Here is a cake I did last year for fall using a lot of leaves.Dawn's Autumn Harvest Cake
Here are a couple of tips:
I used fondant. I cut them out with fall leaf cookie cutters.
I used more than one color in each leaf... that's the biggest thing for a realistic fall look.
I made a little fall red, orange, yellow and brown. I pinch off just a hint of each at a time.Roll them together in a ball without mixing the colors then roll out on your cutting board. See how they look like leaves changing colors? And no two leaves will be alike. I keep rolling that little ball until I can't get anymore leaves out of it. Each time I reroll the fondant the colors merge a little more.
I made veins with the back of a pairing knife because I didn't have a veiner back then. I dusted each with a very watered down luster dust. Just enough for a sheen.
I then crumpled up tin foil in several little balls, allowing the leaves to dry on them. This gave them curves and bends found on drying leavs in nature. They were ready for cake placement the next day.
Here are some sugar cookies I made with the same fondant and rolling technique. These do not have luster dust. This just gives you a better view of the coloring:Fall Cookies