White Diamond

White Diamond can't be countered.
Flash, indestructible, vigilance, protection from instants and enchantments
{x}{2}{w}: Gain control of target creature with converted mana cost X. It becomes white and loses all other colors. If White Diamond becomes tapped, lose control of all creatures you don't own.
At the beginning of your end step, tap White Diamond if you control a permanent that is a color other than white.