Do you need more card ideas for the D&D Adventure System games?
You can start with cheap acrylic paints from Michael's. They work, but they are harder to make look really good. If you just want table colour, they are fine.
I'll second it. I've had pretty good results with craft paints, but they are tricky to work with. I suggest buying an eyedropper to thin the paints with water if you go the craft store route.