The solution that I have found that allows me to both play with watercolors and keep my cardstock and paper warp-free, is to use my watercolors on diecuts that I want to go onto my page. For example, I used my lovely blues and greens to paint this diecut title from the Dear Lizzy collection:
I know this is a very small detail, but it was fun working on the "Best Ever" with my watercolors. And not to mention it was a good little art lesson for my five year as I showed her how to mix colors and layer them to get a specific color. (She worked on a few herself right along side me.)
Isn't art fun??