Instructables user BuildMakeCraftBake has a how-to up on this interesting technique for transferring plant pigments to paper by hammering them. She writes:

The vibrant colors of leaves and flowers are easy to preserve by pounding them to release their natural dyes onto paper. I learned this fun and simple technique from my college roommate, Sarah, but it’s been around a lot longer — I hear that Cherokee women pounded flowers to decorate fabric. With not much more than a hammer and some leaves, you can make beautiful botanical cards or prints and enjoy spring greenery year-round. Who knew that venting your frustrations could have such aesthetically pleasing results?