IANAL but from my understanding, colour of the UI can’t be copyrighted, but the design of the UI as a whole can be. Colour can be a significant factor of the design, but its rarely going to be enough to have enforcable copyright on alone.

Trademark can cover a specific colour, but thats more limitted, generally, and likely wouldn’t cover the colour of a game UI unless its a definining part of the appearance of the game.