Other responses have given some pretty good reasons why spray foam is not a good idea for flotation. I also wouldn't want to more or less permanently block off large areas of the hull that way, even without the other drawbacks.

Assuming that you are mostly looking for an inexpensive way to add flotation, you can use cheap beach balls, inflatable foil covered wine bags, those cheap inflatable 5 gallon water carriers etc. Just about anything that will hold air. For just messing around on flat water it doesn't have to completely fill the area, just provide enough flotation for safety - but you do need some physical restraint to keep it from popping out and floating away just when you need it most. Usually the seat will be enough for stern flotation, and the footpegs keep bow flotation in place.

As for paint, I think most types from house paint to spray acrylics will work. Try it on a small patch to be sure it will adhere and won't do immediate damage, but I've never heard of major paint problems with fiberglass kayak hulls. As for color, there are some things you just have to decide for yourself.