Common Cause

Oracle Text

Nonartifact creatures get +2/+2 as long as they all share a color.

Card Rulings

10/4/2004 It really does mean “all creatures”, including your opponent’s.
10/4/2004 The ability gives +2/+2 to all creatures which are not artifact creatures, as long as every one of those creatures is of the same color. They can have additional colors beyond the one they share, but they all have to share one color. If any creature is not of the shared color, the bonus is lost.

Additional Information