I don't know much about orange shoulder tangs but I vote for transition. Heres a photo I found of one transitioning
You can see on the head how its darkening up some. Neat how it gets its orange "shoulder" first. I would just keep an eye on it. If its due to stress or disease you would see the whole body darkened and the fish might become more lethargic. Just the head makes me think transition.
Then again I could be full of it.
Maybe try wetwebmedia FAQs?