Perhaps both your chemistry and lighting are great, because unless your bulbs are incredibly old or a mile away, you have a lot of wonderful firepower going on. And bright colored corals even existed before amino acid supplements. I think fish poop is still considered the best additive that we know of. Assuming your parameters are reasonable and stable (potassium I believe should be around 390-400 but not sure how reliable hobbyist test kits are and I have no idea of correct range of iodine--I personally dont know anyone who tests for iodine), then that certainly would not be an issue. Again stability is so important. Pests are another consideration but you aren't having any destruction take place, so always possible but not so likely.
Some acros including Millie's are inherently blah. They come in all the colors of the rainbow including feh brown and feh red and feh green. It's easiest to maintain bright colored corals when the frags come from multigenerational tank raised brightly colored mother colonies. That's why red planet, palmers blue Millie, tyree rouge, etc are such good choices because given reasonable stable parameters they always look good . So maybe your corals are the feh ones and may never look more colorful. Genetics is tough to correct ( just ask my kids).