Text (IN): 6/6, Flying. ; Whenever ~this~ deals combat damage to a player, you may pay {2}{U}. If you do, choose a color. Return all creatures of that color to their owners' hands.
You choose the color during resolution. This means your opponent does not get to react after knowing the color you chose. [Invasion FAQ 2000/10/03]
Note - Also see Color, Rule 105.4, for rules on choosing a color.
