You must choose one or more of the five colors of Magic while resolving Scuttlemutt’s last ability. You can’t choose “artifact,” “colorless,” or “chartreuse.”

In the Commander variant, the color identities of cards are determined as the game begins. Changing the colors of a player’s commander doesn’t affect its color identity or the cards that can be legally included in the deck.

The target creature has only the colors Scuttlemutt gives it—Scuttlemutt’s ability overwrites any previous colors the creature had.