In Gears of War 3 for the Xbox 360, you can unlock the Flower Blood mutator, which will replace all blood spatter with pretty flowers, and has no other negative or positive effects.
To unlock the Flower Blood mutator, you need to earn the King of COG medal by earning a total of 2,000,000 points in Arcade mode (this is cumulative).
Check out my other guide if you’re looking for the full list of unlockable mutators in Gears of War 3.
