Here's a drawing I did in my sketchbook of just some leaves outlined with permanent marker. I scanned the image into my computer and then went to town!
I took the drawing and outlined it again in photoshop creating a layer that I placed on top and then worked backwards, adding in the various layers of color. I like how the background makes the leaves pop.
On the second one, I did the same technique with the layers, but changed the outline to white. I built up background layers instead of coloring each leaf.
Lastly, I did the same things all over again, but this time with autumn colors.