10/4/2004: Both of the target creatures must be exactly the same color or combination of colors. Both being colorless is also okay. If they differ in any way, you can still cast the spell, but it does not do anything on resolution.
10/4/2004: Colors are checked on resolution.
10/4/2004: If one of the targets left the battlefield before resolution, use the colors it had at the time it left the battlefield for the comparison.
10/4/2004: If one of the targets became an illegal target before resolution, this spell will still check the color of that target even though it will not destroy that target.