Hey guys! So I am by no means a professional chef here but I've got this awesome idea for my boyfriend's birthday cake. However, it would cause for painting on fondant. My idea is to make a cake that looks like a thing of protein powder and I would need to either paint a good amount of white, yellow, and red on to black fondant or a whole lot of black (also with yellow and red) on to white fondant. So my question is should I do black on white or white on black and what kind of paints should I use to do so?

Have you considered layering fondant on top of fondant?  I usually try to avoid using a lot of black fondant when I can because it usually doesn't taste as good and can be harder to work with, so maybe go with white & then do thin layers of black, yellow and red fondant?  Alternatively you could cover the cake in black & then do a thin layer of white & then paint the yellow and red on the white.  To paint on fondant I mix americolor with a bit of vodka.  Don't forget to use a paint brush that has never been used for anything but food, and have a piece of fondant nearby to test your "paint" on before you go to work on your cake.  Hard to picture exactly what you have in mind without knowing what the protein powder jar really looks like, but hope this helps!