Just to add - I've heard of multiple different tang species either keeping or reverting back to their juvenile coloration well into adulthood/transition, including these tangs, orange shoulders, and Atlantic Blues. It seems to have something to do with confidence/dominance of the tang (it's most commonly seen when there's another, more dominant tang in the tank, but it's sometimes seen in cases with no other fish in the tank, when the fish is in QT/an acclimation box, etc.).
When the fish feels confident, it should finish transitioning to adult coloring.