The eternal question with prot-colour tricks is how to stop people using them on opponents' creatures to remove their beneficial auras. The usual approach is to let the creature's controller choose the colour that the creature gains protection from (which works on everything except Genju of the Realm). This makes it really hard to do a several-target version of that kind of spell, but I think this wording is a reasonable compromise.

Is it too problematical to have "up to two target creatures gain protection from a colour of their controllers' choice until end of turn" let a player choose two different colours?

The problem is that it's very unclear whether or not it lets a player choose two different colours. It probably does, but I'm not even sure.

This name is rather bad - it's a bit like "Angry Rage" or "Quiet Peacefulness".