All creatures that share a color are affected, even your own.

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.

You check which creatures share a color with the target when the spell resolves.

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

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.

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.