All the Coop and Migros Baumarkt have all types of paint. The selection of colored wall paint is quite limited though. I think you can buy small packs of color though that you mix with white to get the color you want. In any case, get hold of a shop attendant (good luck with that one!) and tell them what you want so that you get the right stuff.

And as others have mentioned, you'll have to repaint back to the original color before you leave if you're renting.