Common Cause | Magic: The Gathering
Formats Common Cause is Legal in
Official Oracle Text for Common Cause
Official Rulings for Common Cause
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.