Cora

If a spell or effect controlled by an opponent
targets Cora, counter it unless that spell's
controller pays {1} for each of its colors.
{1}: Until the end of turn, all other permanents
and spells become the color of mana spent to
activate this ability in addition to their other
colors, and all lands can tap to produce one
mana of that color.
If Cora is your commander, your deck can
contain any color.