Crinkled masa paper plus watercolor provides some interesting effects.
I've done 4 paintings now, experimenting with this technique, and I have to say I'm really liking the effects you can get. The peonies pictured below turned out really soft and transparent-looking. Almost as if I'd used layers of colored tissue paper to make the image. (Hey - that's another interesting idea!)
The irises pictured above have a different feel - almost diffused, or impressionistic. For the irises I painted on the back side of the masa paper, which has a slightly fuzzy texture. This would be the explanation for the diffused effect of the painting overall.
The tulips are another idea again - the spread of color into the background gives a feeling of movement that I wasn't expecting. All in all, a fun exercise and I like the experimentation!