"Beautiful, isn't it?" crowed Crovax. "They fought together. Now they will die together."

Dead Ringers

Destroy two target nonblack creatures unless either one is a color the other isn't. They can't be regenerated.

Mystery Booster 2

Apocalypse

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.