I had my Picasso for 5 years. GREAT FISH, mine would play fetch using a pingpong ball.... Anyways, I'd first check the water parameters, as is always a good starting point. Than I'd look at how well you're providing a varied diet. One thing I noticed w/ my Picasso when it got up to 1' or so, is that the fish excreted a lot. Meaning my filtration had to deal w/ a large bio load spike on a regular base. Look into improving your biological side of the filtration, and dont skimp on the mechanical side of the filtration. You'll see what I mean when they get large
Great fish, and I'm sure yours will get some great color yours is really young... Mind started getting great dark color in his bands and the strong blue across the eyes at about 3-4".
One problem w/ the fish trade is that "Picasso Triggers" aren't always the Picasso you think. There are quite a few trigger fish that are labeled by pet stores as Picassos, such as the Rhinecanthus aculeatus / Huma Huma which is the most commonly sought after Picasso, I've even seen Sufflamen bursa sold as a picasso but in fact, Rhinechanthus rectangulus (rectangle trigger) is the true Picasso.
Mine was the aculeatus, and was very beautiful.