I got a little carried away with the details on this tree. When I started, I made the top part too small for the trunk and ended up having to fill it out to make it look right. As a result, the leaves got much more detailed-- which I like a lot. I added some simple roots for interest.
For this tree, I used Cadmium Yellow Hue, Cobalt Blue, and a little Lemon Yellow Hue. I also painted the trunk with Cobalt Blue first and added a layer of Dark Brown over the top-- just for a little dimension. I really like how the yellow of the tree bleeds down into the branches, too.