Sailfin tang are pretty common. Usually the ones I see are a dark brown with vertical yellow bars and a yellow tail. A close look reveals thinner pale stripes between the vertical bars. There’s a photo here.
These two sailfin tangs, that I saw in shallow, murky water, were a first for me. They’re much paler and brighter, the thinner stripes are clearly defined, and their dorsal and anal fins are extended.
According to my fish book, the fins extension could be because the fish were alarmed, but it doesn’t say anything about them become lighter when this happens. I wondered if it was some sort of mating coloration, but again, my book says the fish darken when they do that.
So I’m not sure what was going on, but was happy to have spotted them and pleased that I got a decent photo given the murky water conditions.