I don't check no3 or po4 ever so I can't comment on that.
However, I do believe that lighting plays a big role in sps coloring. I've been running led, t5, and mh simultaneously in my frag tanks in the last 4+ months. All the corals share the same water so that eliminates water quality out of the equation. I've noticed the same corals color up differently under each light. Some corals look better under certain lighting. Some look like poo.