Use leaves in a collage,<br />
Smear same colour (clean) leaves against the paper / canvas,, or try using a blender.
Try painting with berries.
Beets will give you a nice red purple. Onion skins will give you a nice golden color. We used natural plants to color our Easter eggs last year. Many of them didn't work well. I found a site for you, How to make natural paint from plants. Asian Fanatics Forum.