I find that the color of the adults is highly variable and have seen full adults ranging from a nearly black color to a mustard yellow and every shade in between. There are three species of mimic tangs and all the adults look similar with slight differences in coloration. I think the tristus (Eibl angel mimic) mimic tang is the most drab as an adult, most of the ones I have seen were a grey color.
The last mimic I kept was a Herald's angel mimic (all yellow juvie) and was about 1 1/2 inches when I got her and the adult coloration cam in at around 3 1/2 inches and was a mustard yellow body, greenish fins with a stripe of bright yellow through the tail and orange markings behind the gills and on the head and face, just a beautiful species. I would find a healthy juvie if you can so you can document the color change to adulthood, its one of the perks to keeping this species.
IMO they are one of the hardier tangs of the genus and because of their smaller size and better disposition than most tangs in general are a great choice for community tanks.
Here are pics of a small juvie, the intermediate and the full adult, same fish pyroferus mimic tang.