You could just paint over it, but I'd suggest using a primer first, or at least painting your wall white before the pink and green. The primer helps your new paint stick to the wall better and look better after you paint. If you paint the wall white before you paint it pink and green, it will be hard to tell that your wall was ever blue, and it'll be like starting off on a clean slate. Even if you can still see the blue, you can just paint another layer of the pink and green. Hopefully this helps you, even in the smallest bit. =]