Common Cause — ruling 1

It really does mean “all creatures”, including your opponent’s.

Common Cause — ruling 2

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.

Common Cause : règles, fonctionnement et interactions (MTG)

Common Cause is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

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

How it works : Common Cause

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2004· Wizards of the Coast
It really does mean “all creatures”, including your opponent’s.
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
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.
No other card is named in this ruling.