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 gain life equal to the total damage dealt by Brightflame to all creatures. You don’t gain life for any damage that was prevented.

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

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

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.

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.