Text (MM): Nonartifact creatures get +2/+2 as long as they all share a color.
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. [MM FAQ 1999/09/22]
It really does mean "all creatures", including your opponent's. [MM FAQ 1999/09/22]
