If you have a large ink pad that would be nice and simple. I used a folded paper towel covered thinly with washable paint. A plastic lid is under it (for protection).
The blue on blue turned out well. I noticed the foam doesn't hold a lot of paint, so the stamps are rather light, but still leaves a nice picture behind.
This is fairly simple to make. If you don't have access to wood scraps, I'm sure a small sturdy box or glued together cardboard would work too. In the past I used a wooden ABC block from the toy chest. The paint absorbs into the wood, though, so I don't recommend that. :)