I had a single spot several years ago, and he would drop his yellow and goto that molted grey/brown/yellow color at night, during feedings, and often during the day....He was a good eater and peacful, I kept him for a year or 1.5, but one day my yellow tang accidently ran into him during feeding and got stung/venomated and went into spasims/seizures and died within 10 minutes. I returned the siganus unimaculatus to the lfs, cause I prefer tangs - and watching that poor yellow dude twitch & flop around and go into shock then drown kinda put me off rabbitfishes. They were in a 100g tank, but a collision like that could happen in any size tank, and the tang will loose that battle everytime......maybe it was just my bad luck too? As long as he eats, he'll be ok, but remember he'll not only sting/venomate you - he could do it to tank mates - even if it's accidental...
And yes, they'll drop their yellow in seconds and mine did quite often, sometimes for no appearant reason during the day, but be normal again within minutes...IME
Edit - If I remember right, he was kinda a bottom/mid water column feeder - not so much top, but would take food from a clip after a while. I had him in '03 or so and don't remember all the details, but I sure remember why I returned him! though I never got stung, but have heard from several who have, and it does not sound pleasant!
