Understanding Color Theory Excerpt from Fundamental Photoshop by Adele Droblas Greenberg and Seth Greenberg

Transcription

1 Understanding Color Theory Excerpt from Fundamental Photoshop by Adele Droblas Greenberg and Seth Greenberg Color evokes a mood; it creates contrast and enhances the beauty in an image. It can make a dull scene vibrant and a tired image suddenly sparkle with life. 1. What is Color? 2. The RGB Color Model 3. The CMYK Color Model 4. The HSB Color Model 5. The LAB Color Model 6. Other What is Color? Color exists because of three entities: Light The object being viewed And the viewer Physicist have proven that white light is composed of wavelengths of red, green, and blue that are absorbed or reflected by objects. For example, assume you are at a picnic on a sunny day, ready to reach for a red apple. Sunlight shines on the apple and the red wavelength of light is reflected off the apple back to your eyes. The wavelengths of blue and green are absorbed into the apple. Sensors in your eye react to the reflected light, sending a message that is interpreted by your brain as the color red. Your perception of the red color depends upon the apple, the light, and you. One apple will absorb more green and blue than another, and thus its color will appear redder. If clouds cover the sun, the apple s red will appear darker. Your interpretation of the apple will also be affected by your own physiology, by your experience as an apple eater, or by the fact that you haven t eaten all day. The red, green, and blue wavelengths that allow you to see the apple are the basis for all colors in nature. That is why red, green, and blue are often called the primary colors of light. When the three primary colors overlap, they create the secondary colors: cyan, magenta, and yellow. The primary and secondary colors are complements of one another. Complementary colors are colors that are most unlike each other. Yellow is made up of red and green. Blue is the missing primary color; therefore, blue and yellow are complements. The complement of green is magenta, and the complement of red is cyan. This explains why you see other colors besides red, green and blue. In a sunflower, you see yellow because red and green wavelengths of light are reflected back to you, while the blue is absorbed by the plant. All primary colors combine to create white. You might think that adding these colors together would produce a darker color, but remember that you are adding light. When light wavelenghts are added together, you get lighter colors. This is why the primary colors are often called additive colors. By adding all of the colors of light together, you obtain the lightest light: white light. Thus when you see a white piece of paper, all of the red, green, and blue wavelengths of light are being reflected back to you. When you see black, all of the red, green, and blue wavelengths of light are being completely absorbed by the object; thus no light is reflected back to you.

2 The RGB Color Model The system of creating colors on your monitor is based on the same fundamental properties of light that occur in nature: that colors can be created from red, green, and blue. This is the basis of the RGB color model. Your color monitor creates colors by emitting three light beams at different intensities, lighting up red, green, and blue phosphorescent material overlaying the inside of your monitor s screen. When you see red in Photoshop, your monitor has turned on its red beam, which excites red phosphors, lighting up a red pixel on your screen. Thus, seeing a scanned image of an apple on screen is different from seeing the apple that sits on top of your computer, waiting to eaten. If you turn off the lights in your room, you won t see your red snack; but you ll still see the scanned apple, because light is being emitted from your monitor. In Photoshop s RGB color model, pixel colors can be changed by combining various values of red, green, and blue. Each of these three primary colors has a range of values from 0 to 255. When you combine the 256 possible values of red, 256 values of green, and 256 values of blue, the total number of possible colors is approximately 16.7 million (256x256x256). This may seem like a lot of colors, but remember that these are only a portion of the visible colors in nature. Nevertheless, 16.7 million colors is sufficient to reproduce crystal clear digitized images on a monitor equipped with 24-bit color. Using RGB Colors in the Colors Palette If the Colors palette isn t open, open it now by choosing Show Colors from the Window menu. By default, the Colors palette will display the RGB color model palette, unless it was changed in a previous session. Tou should see three bars, called color sliders, label R, G, and B. If you do not see the letters R, G, and B, you are in another color model s palette. To change the RGB palette, click on the Colors palette Option menu arrow. A list of Photoshop s color models will appear. Select RGB Color from the list, and the palette with the three RGB sliders will load. Beneath each slider is a triangular-shaped slider control. Using the mouse, you can click and drag on the slider controls to change the values of red, green, and blue in the foreground or background color. The foreground and background colors are indicated in the selection boxes (the overlapping squares in the upper-right corner of the Colors palette). Using Swatches to pick Colors Before you begin, make sure that the Foreground selection box (the top overlapping square to the right of the sliders) is activated. If the white band is not surrounding the Foreground selection box, click on that box to activate the foreground color. To see the RGB values of the primary color red, position the mouse pointer over the red color swatch in the upper-left corner of the color swatches, in the first row. When the pointer turns into an eyedropper, click on the red swatch. Notice that the R slider control jumps to the far right. The number to the right of the slider indicates the red value of the color swatch you just clicked. (If you clicked the correct shade of red, the value will be 255). The active color in Photoshop (the foreground color) has now changed to this value of red. Take a look at the sliders for green and blue; are both at 0, because the shade of red you clicked contains no green or blue. To see the RGB values of the primary color green, click on the green swatch (the third from the left in the first row). The G value jumps to 255, and the values of blue and red

3 drop to 0. To see the values of the primary color blue, click on the fifth swatch from the left in the first row. Pure blue s value is 255, with no red and no green. You can see from the foreground color swatch tests that each primary color has a range values from 0 to 255, to produce 256 separate color values. Using Sliders to create RGB Colors Click on any swatch, and you ll see its combination of RGB color values. Try the yellowish color, the one directly below the pure blue swatch. Notice the combination of values: R 255, G 236, and B 103. Click on a few of the other color swatches, and notice the combinations of the three primary colors. The palette swatches obviously can t contain all 16.7 million colors, but by adjusting the slider controls with the mouse, you can create every one of the 16.7 million colors Photoshop can display. All three sliders work in the same manner. When you click and drag a slider control to the right, you are adding more color. When you drag left, you are subtracting color. To create black, drag all the slider controls to the left, setting them to 0. If red, green, and blue are equal in value but are not 0 or 255, a shade of gray is created. To create white, move the slider controls in the opposite direction. Set each slider to its full value of 255, and you ll create white. Using the Sliders to Display RGB complements If you are using the Dynamic Sliders option, you ll see a range of colors displayed in each slider when all of the RGB sliders are set to 255. The R slider displays white turning to cyan; the G slider displays white changing to magenta; and the B slider displays white changing to yellow. In each slider, you see the opposite of each primary color (its complement). To create yellow, which is the complement of blue, drag the B slider control back to 0, and leave the R and G sliders set to 255. Thus yellow is created by subtracting blue from white. To create cyan, which is red s complement, first set all sliders back to 255, and then drag the R slider control to 0. By subtracting the red value from the primary colors, you have created cyan. To create magenta, the complement of green, drag the R slider control back to 255 and the G slider control to 0. Magenta is created by subtracting green from the primary colors. The CMYK Color model The CMYK color model is based not on adding light, but on subtracting it. In the RGB model, colors were created by adding light; the monitor ( or a television) is a light source that can create colors. But a printed page doesn t emit light; it absorbs and reflects light. So when you want to translate the monitor s colors to paper, the CMYK color model must be used. The CMYK color model is the basis for four-color process printing, which is used primarily to print continuous tone images (such as digitized photographs) on a printing press. In four color process printing, colors are reproduced on a printing press by using four different printing plates: C (cyan), M (magenta), Y (yellow), and K (black black is represented by the letter K because B might also stand for blue). Because the printed page cannot emit light, a press cannot use RGB colors to print; instead, it uses inks that can absorb specific wavelengths of light and reflect other wavelengths. By combining inks of cyan, magenta, and yellow, a commercial printer

4 can reproduce a significant portion of the visible spectrum of colors. In theory, these three colors should combine to produce all colors necessary for printing, including black. However, because of the impunity of inks, the cyan, magenta, and yellow colors produce a muddy brown rather than black. Therefore, printers often add black to cyan, magenta, and yellow to produce the darker and gray portions of images. Using CMYK Colors in the Colors Palette. First, you ll need to switch to the CMYK color model by clicking on the Option menu arrow in the Colors palette. You ll immediately notice some differences between the RGB and CMYK palettes. First, CMYK utilizes four sliders, not three. There is a slider for each subtractive color: C for cyan, M for magenta, Y for yello, and K for black. Also the colors are measured in percentages. The percentages provide a standard way of conveying various ink values from the design studio to the printing press. The color wheel is a simplified version of the color wheels professional color correctors use to help them add and subtract colors. In a color wheel, colors are arranged in a circle to show the relationship between each color. The primary colors are positioned around the circle equidistant from one another. Each secondary color is directly opposite its complement, and each color on the wheel is situated between the two colors that are used to create it. By studying the wheel, you can see that adding yellow and magenta creates red. If you subtract both yellow and magenta, you remove red. Thus, if you want to subtract red from an image, you decrease the percentages of yellow and magenta. Another point to realize about the color wheel is that when you are adding color to an image, you are subtracting its complement. For instance, when you are making an image redder, you are decreasing the percentage of cyan (which is red s complement, directly opposite it in color wheel). Using Sliders to Create CMYK Colors Now you ll practice using both the color wheel and the CMYK sliders to create red, green, and blue. This simple exercise will demonstrate how CMYK inks absorb different light waves and reflect others to produce colors. To create red, drag magenta and yellow to 100%, and leave cyan and black at 0%. Notice that red is between yellow and magenta on the color wheel. When you see red in an image printed using four-color process printing, yellow and magenta inks have combined to absorb the blue and green wavelengths of light. The result is the color red. To create green, drag yellow and cyan on the color wheel. When you see green in an image printed using four color process printing, yellow and cyan inks have combined to absorb the blue and red wavelengths of light. To create blue, drag cyan and magenta to 100%, and yellow and black to 0%. Blue is between cyan and magenta on the color wheel. When you see blue in an image printed using four-color process printing, cyan and magenta inks have combined to absorb red and green wavelengths of light. The Out-of Gamut Alert The color ranges of the RGB and CMYK color models are different. The professional term for the visible color range of a color model is gamut. The gamut of RGB is larger than that of CMYK. Therefore, if you only work with RGB color on your computer, be aware that you maybe designing and editing with onscreen colors that cannot be printed.

5 Fortunately, Photoshop provides a warning when you overstep the bounds of printable colors. You may even have already noticed this warning; it s a small alert symbola triangle with an exclamation mark inside-that appears in the colors palette. Even when you are working in the CMYK palette, you can still activate the alert feature by clicking on any of the first six color swatches from the left in the top row. For example, click on the red swatch in the top row of the colors palette. The alert will immediately appear, indicating that this color is beyond the CMYK gamut. If you click on the alert, Photoshop will select the nearest printable color. Try clicking the alert now; the closest printable red will be displayed as the foreground color. But even though Photoshop is showing you that the red is printable, it is only displaying and RGB monitors s simulation of red. Never assume that the color you see on screen is an exact representation of the printed color. Note: If you are outputting your works to slides or video, you needn t worry about the out-of-gamut alert, because slides and video use RGB colors. The alert indicates that the color cannot be created by commercial printers inks. The HSB Color Model HSB is based on human perception of colors, rather than the computer values of RGB or printers percentages of CMYK. The human eye sees colors as components of hue, saturation, and brightness. In technical terms, hue is based upon the wavelength of light reflected from an object. or transmitted through it. Saturation, often called chroma, is the amount of gray in a color. The higher the saturation, the lower the gray content and the more intense the color. Brightness is a measure of the intensity of light in a color. Using HSB Sliders in the Colors Palette Let s take a look at how to choose colors using the HSB color model. In the Colors palette, click on the Options menu arrow and choose HSB color. You ll see three sliders: H for changing hue values, measured in degrees. S for saturation, B for brightness, both measured in percentages. Try this now by clicking and dragging the H slider control to the right. As you move through different degrees of hue, the foreground or background selection boxes in the Colors palette will display the same colors that you would see if you were traveling the perimeter of a color wheel. When you reach 360 degrees, you ll see the same red that is displayed when you are at 0 degrees because you are traveling around the circumference of a circle. The major color values of hue, with saturation and brightness set to 100%, are as follows: Color Hue Setting Red 0 to 360 Yellow 60 Green 120 Cyan 180 Blue 240 Magenta 300 Start by selecting the red color swatch in the first row of the color swatches. Move the S slider control. The intensity of the red will diminish as you decrease saturation by

6 dragging the slider control to the left. Try it, and you ll move from red to light red to pink. When you reach 0%, the red will have changed to white. Note: You cannot create white and black with the hue slider. To create white, the saturation slider must be set to 0% and the brightness slider control to 100%. To create black, the saturation slider control must be set 0% and the brightness control to 0%. Before you test the brightness slider, click and drag saturation back to 100%, to give you a reference point for observing the effects of changing brightness. Dragging the brightness control to the left will decrease the percentage of color brightness, dragging to the right will increase the brightness. The LAB Color Model LAB color consists of a luminance or lightness component (L) and two Chromatic components: The (A) component, which ranges from green to magenta, and the (B) component, which ranges from blue to yellow. LAB color model is vital to Photoshop. LAB is the internal color model that Photoshop uses to convert from one color mode to another. When Photoshop converts from RGB to CMYK, it first converts to LAB color, then from LAB to CMYK. One reason why it uses LAB is because the LAB color gamut encompasses both the color gamuts of RGB and CMYK. The LAB color model is based upon the work of the Commission Internationale de l Eclairage that was formed in 1931 to try to standardize color measurement. The commission designed a color model based upon how color is perceived by the eye. In 1976, the original color model was refined and called CIE LAB. It was created to provide consistent colors, no matter what monitor or printer is being used: This is called device independent color. Device independent color isn t affected by the characteristics or idiosyncrasies of any piece of hardware such as the monitor, printer or computer, that you use to create or output the image. The LAB mode is used most often when you are working with Photo CD images or when you want to edit the luminance and the color values in an image independently. LAB color is also the recommended color mode for moving images between systems and for printing to PostScript Level 2 printers. To print LAB images to other color PostScript devices convert the images to CMYK before printing. Using LAB sliders in the Colors Palette Click on the Options menu arrow and switch to LAB color. You ll see three sliders, label L, a, b. The L slider allows you to change a color s lightness, in values from 0 to 100. The a slider represents colors ranging from green to magenta The b slider represents colors ranging from blue to yellow. Start with the L slider control set all the way to the right at 100. This provides the brightest levels for the colors you will be creating. To see how the a slider changes colors from green to magenta, set the b slider to 0. Then, click and drag on the a slider control to the left, the color will turn to green. Now drag the a slider control to the right to see the color change to magenta. When you are done, leave the control in the middle of the a slider (set to 0). This will best enable you to see the effects of changing the b slider. Now observe how the b slider changes colors from blue to yellow by clicking and dragging the slider control. First drag it to the right, and the color will turn to yellow. Drag to the left to see the color change to blue.

7 Other Color Model Bitmap Mode and Pixel Depth Bitmap mode uses one of the two color values (black or white) to represent the pixels in an image. Images in bitmap mode are called bitmapped, or 1-bit images because they have a pixel depth of 1. Pixel depth, also called bit resolution or color depth, measures how much color information is available for each pixel in an image. Greater pixel depth means more available colors and more accurate color representation in the digital image. For example: A pixel with a pixel depth of 1 has two possible values: black and white. A pixel with a pixel depth of 8 has 256 possible values; and a pixel with a bit depth of 24 has roughly 16 million, possible values. Grayscale Mode Grayscale mode uses up to 256 shades of gray to represent an image. In Adobe Photoshop, every pixel of a grayscale image has a brightness value ranging from 0 (black) to 255 (white). Images produced using black-and-white or gray scale scanners are typically displayed in Grayscale mode. Indexed color Mode An indexed-color image is based on a palette of at most 256 colors. When you convert an image to indexed color, Photoshop builds a color lookup table (CLUT), which stores and indexes the colors in the image. If a color in the original image does not appear in the table, the program matches the color to the closest color in the color table or simulates the color using the available colors. Indexed color mode is useful when you want to limit the palette of colors used in an image. For example, when you want to use the image in a multimedia animation application or on a Web page.