Text (PS): 2/2. ; {2}{W},{Tap}: Choose a permanent you control. Prevent all combat damage target creature would deal this turn if it shares a color with that permanent.
You choose the target on announcement and choose the permanent which is compared for color on resolution. [Planeshift FAQ 2001/01/26]
You only check colors on resolution and not later when the damage prevention actually is applied. [Planeshift FAQ 2001/01/26]
