Common Cause

• WWizards of the Coast2004-10-04
  Official Ruling
  It really does mean “all creatures”, including your opponent’s.
• WWizards of the Coast2004-10-04
  Official Ruling
  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.

Rulings via Scryfall