Cheapest priced copy (Mercadian Masques): $0.59 at Card Kingdom, near mint, their price list as of .
Common Cause 
Enchantment · Rare · Mercadian Masques · #13
Nonartifact creatures get +2/+2 as long as they all share a color.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $0.59 to $7.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Mercadian Masques | 13 | Rare | $0.59 | $7.99 |
2 rulings from Wizards of the Coast
- 4 October 2004It really does mean “all creatures”, including your opponent’s.
- 4 October 2004The 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.