Usually loss of colour is a sign of stress, my achilles looked like a powder brown hybrid (greyish) the whole time he was in quarantine but the day after he went into the display tank his colour came back. He may still be expecting the regal to jump out of the rocks at him and may take a while to feel comfortable in his new surroundings.
Do you have any other fish in the tank that is bigger then him that he might feel a bit threatened by?
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