2005-10-01 : A creature “shares a color” with any creature that is at least one of its colors. For example, a green-white creature shares a color with creatures that are green, white, green-white, red-white, black-green, and so on.

2005-10-01 : All creatures that share a color are affected, even your own.

2005-10-01 : If it targets a colorless creature, it doesn’t affect any other creatures. A colorless creature shares a color with nothing, not even other colorless creatures.

2005-10-01 : Only one creature is targeted. If that creature leaves the battlefield or otherwise becomes an illegal target, the entire spell doesn’t resolve. No other creatures are affected.

2005-10-01 : The color you choose as the spell resolves has nothing to do with the color or colors shared by the affected creatures.

2005-10-01 : You check which creatures share a color with the target when the spell resolves.