I agree with the phosphates or lighting creating your problem - what lighting are you using? pc's, t-5's, mh? what kelvin?
As far as nitrates go, 15ppm should not have any effect on color, and unless you have a truely hard to keep coral it shouldnt "kill" it either. Oppm of course is what everyone shoots for - however some corals prefer water that is a little "dirty" Xenia and Zoanthids for example. I have nitrates at levels of 15-30 because I tend to over feed my tank and at this point do not have a sump because of tank limitations. I do however keep softies, LPS and yes SPS in this tank nicely and see significant growth on all.