Mine is like that at night or after it's been in the dark for a long time. After the lights come on he seems to color up. My mollies and bumblebee gobies (to some extent) do this too.
The bristlenose pleco in with my dragon goby turned near pure white when the lights were left off for a few days while on vacation. He turned black again within 15 minutes of having the lights on.
Mine has never been a true violet hue. He is more blue-ish to me. We have had him for about a year and a half. He's a little over 11 inches now and really fat. I really haven't seen him change colors, but he loves to swim in the jet we have in his tank. Oh, his name is Snakeman.