Well, to be honest, I don't really see a need to add a Rainbow Pickaxe right now, especially since the Diamond Pickaxe is already serving it's purpose as the highest tier pickaxe for mining. If anything, you can suggest to add a Recipe which allows you to create a Rainbow Pickaxe from Crafting, if that's what you were trying to say, which again, I don't really see a need for it besides giving players a fancier pickaxe.