Could be a bit of the stress, especially if they just took them out of the bag, could be diet, could be tank water that is slightly cloudy, could be that most pictures of what you see are deliberately made to look fantastic with colors that pop. I mean just looking at the yellow tang in the picture it doesn't look anywhere close to as yellow as you see in pictures. But I most definitely see the yellow around the edge of the eye, it might not be as bright, but it's there.
One of the most important thing to worry about with bristletoothed tangs IMO is the mouth, since they scrape at the rocks for algae (they do eat fish food too), if their mouth is injured/infected you might want to pass. Not saying the one in the picture is, it's hard to tell, but it does look a bit suspect (could be the angle of the photo too)