I know that in order to get the Pink'd acheivement you need to have "head, shoulder, feet, leg, hand, and torso equipment colored with Lovely Dye", and I have of course done that - but I notice as I play through the game I sometimes get credit towards the acheivement for killing bosses, and other times I don't. Am I doing something wrong? I'd suspect it was bugged, but since I sometimes get credit it can't be completely broken (or at least that seems unlikely).
It could be possible that any followers/allies of yours actually deal the killing blow to the enemy, thereby taking the kill and achievement progress from you. It would seem like an arbitrary reason if not for the achievement that deals with letting your followers get kills on certain bosses, 'I'll Be Your Wingman'.
EDIT: It would appear that followers do not block achievement progress (if they get the kill) when you're in pink armor, however other players will. After the testing we both have done (same results as you had), it seems like this is pretty set in stone. Go solo, get achievement, win at life.