When painting a tree with leaves you want to begin by painting the "bones" of the tree. This means to paint the trunk and a large number of branches to create a base layer for the tree. Begin by painting the trunk of the tree. Using a liner brush, create a thick line. I paint the base trunk layer with a mixture of Payne's gray, and Raw Umber. Dilute the paint mixture with water to allow for a smoother consistency, and the ability to paint longer brush strokes. Try to put a very slight angle on the tree just to create some interest.
Once you create the trunk, you want to start adding some branches. Begin by adding a few thick branches that split off in a few directions from the main trunk. Using the liner brush just make some organic lines that split off in different directions. Make sure they get thinner as you move further away from the tree.
Continue to add smaller and smaller branches that come off the larger branches. When doing this, try and envision what the shape of the tree will be. Imagine what it will look like with leaves. This will be our guideline when adding the leaves. I am now going to begin adding the leaves. Mix some Sap Green and Payne's gray. The mixture should be more on the green side. We will make the base layer of the leaves. When adding the leaves use a flat, round of filbert brush. When loading the brush, tap the brush into the paint. Then when you add the leaves to the tree, tap the brush on the canvas. This will spread the bristles out and create a patter that resembles leaves. The tapping will create small spaces along the edge of the leaves.
Continue tapping around the tree. You can leave some spaces in the tree where you can see through to the sky. Just try and create a pattern that would look like a tree you would see in nature. Try not to make the tree to round or uniform. Once you have completed the base layer. Allow it to almost dry. While it is drying we will add highlights to the trunk and branches. With a mixture of raw umber, white, and a little Payne's gray we will add some highlights. With your liner brush start on the edge of the tree and work in the light color. Add darker raw umber and gray as you move away from your light source.
Try to add a lot of variation in the trunk. Don't just paint solid lines of color, make little hash marks, and variations as you move down the trunk. You can even add a little red, or green in your raw umber mixture to create some color. Continue to add brighter highlights on the edge of the trunk and branches. You can be subtle or you can make the branches bright and washed out. In this painting I want more contrast so I have made the branches subtle. You can still see the highlights, but the trunk and branches remain dark. We are now ready to add some highlights to the leaves to finish the tree.
Go back to your sap green mixture and add some yellow green. This will create our leaf color. Tap the brush into the mixture again to load. Then start where you light source is, at the edge of the leaves. Tap into the leaves going away from the light source. Make sure to leave areas of dark to create shadows and contours within the tree. As you work away from your light source the leaves should get slightly darker. Some areas of the tree will be brighter than others. Think about where the tree has branches sticking out catching light.
Once you have the base layer of light green down, you can begin adding highlights. You can add brighter highlights to the edges of the contours to give a 3 dimensional effect. Just make sure to leave areas of dark on the opposing side. This will create shadows and make the highlights stand out more. Work your way across the tree, going away from the light source. This will allow you to recognize where to put shadows. Make sure to have the highlights be darker as you move across the tree, this will give a rounded effect.
I like to go back in with a liner brush and add a few highlights to the branches. This will also help the trunk stand out a bit from the sky. Just add a few highlights here and there. You have completed a tree with leaves.