Blah coral colours... any tips?

[doch]

So it seems that I can't get good colouration out of a fair number of the sps that I put in my tank. It's a 55 gal with an Aquaticlife 6 bulb T5HO fixture... If I remember corrctly, 2 fiji purples, 2 super blues, and 2 coral lite 2's. Light cycle of 13 hours on the super blues, and 12 hours on the rest. Chemistry... phospates are next to zero, nitrates are clear as water, amonia and nitrite right around zero... haven't been testing for a while as they were steady zero for every test. Calcium is right around 420-450. Magnesium is 1325-1350. Alk is about 10dKH (fluctuates a little) and pH of about 8.2. So, as far as I can tell, all of my parameters are right around where they should be, and to me I have adequate light (maybe not the best, but should be good) and I seem to loose the nice colours. As an example, I picked up what would have been my favorite frag from the LFS, and left town for work for 4 days... it started off as a deep green with nice purple tips... when I got back in town it was a creamy light brown, with a little bit of purple on the tips. Some stuff colours up, some maintains colour, but for the most part, SPS seem to lose their pop in my tank. I also don't get nearly as much growth as others seem to... sold a couple of frags that had lost their colour in my tank to a guy on here, my similar corals have hardly grown at all, he says his have doubled in size.
I feed Mairine snow, pods, phyto, and another one that I can't remember the name of right now... concentrated coral food or something like that (lottle bottle with a dropper).
What am I doing wrong??? Did I buy a junk light?

Thanks all.