Most people don't realize that acrylic paint will actually stay on something like an iphone case. I'm sure this depends on the materials it is made out of, but for my plastic/ rubber like case, it works great!
Oh and for taking it off and painting it again... I just got some close to boiling water and soaked the case in it for about 15 minutes or so. after that its pretty easy to scrape off with a rough brush or something (or just your nails if you are as unprofessional as me :p ).
This is how I painted it the last time. My favorite thing about this project is that you can keep doing it. This is my third time to do it and I only change it because I get bored easily (; And it lasts for a long time (at least I think. The first time I did it I kept it on for a few months and it never faded).