If 7 colorless mana can destroy any permanent, can an expensive enough mono green spell have both deathtouch and fight?No. Just because we allow colorless to do something doesn’t mean we let colored things do it if it breaks color pie (or feels inappropriate). For example, we’ll make 7 “destroy target permanent” and not make 6R “destroy target enchantment”.
It’s an important that cards in a color don’t do things that color doesn’t do even if you can still play a colorless card in its place. Once you see a red card with the words “destroy target enchantment” it conveys the color can do something it can’t.
