The background has been made with colored crepe paper. When you wet it when laid on other cardstock, it will color the cardstock very randomly, and also blend the colors.
I splashed some white with a white acrylic marker - one that doesn't write that well anymore, but still gives splashes.
I am entering this in the following challenges:
1. IndigoBlu challenge - spring is in the air!
2. Stampotique SDC245 - Circles