Sword of Truth and Justice

Protection from a color means that the equipped creature can't be blocked by creatures of that color, can't be the target of spells of that color or abilities from sources of that color, can't be enchanted or equipped by Auras or Equipment of that color, and all damage that sources of that color would deal to it is prevented. Nothing other than these events is prevented or illegal.

If a permanent or player you choose while proliferating has more than one kind of counter on it, it will get one more of each of those kinds. This means that if you put a +1/+1 counter on a creature you control with a -1/-1 counter on it, then choose that creature while proliferating, you'll put another of each of those counters on it before state-based actions remove them.

If a player or permanent has more than one kind of counter on it, and you choose for it to get additional counters, it must get one of each kind of counter it already has. You can't have it get just one kind of counter it already has and not the others.