My SPS are either losing color or turning fluorescent green

I have been keeping SPS corals in my 150 gallon reef tank for a little
over two months since installing a 250 watt HQI minipendant with a 10K
bulb installed, using a icecap 250 Watt electonic ballast. The
distance from the middle of the bulb to the water surface is about 6
inches, with a UV shield preventing water splatters from bursting the
bulb. What I have been noticing is that the majority of my SPS corals
have either been losing their colors, or turning a fluorescent green
color. All the SPS that I have placed in the tank appear to be
healthy with good polyp extension, but once again the colors appear to
gradually fade. My nitrate and phosphate levels are zero. When I
first added SPS, I placed them high up in the tank within 10 inches of
the MH bulb, but virtually every single SPS appeared to begin to
bleach at this distance (this includes a brilliant blue A. tortusa
frag). When I noticed this bleaching, I moved the majority of frags
further down in the tank to about 16" from the bulb. This includes
the A. tortusa frag, which has made a great comeback and is now once
again a brilliant deep blue. Since I know that the A. tortusa frag is
considered one of the most light demanding species, I am guessing that
I need to move my other frags even lower in the tank. Further I am
wondering that perhaps changing my bulb to a 250W XDE 20,000K HQI may
help the coloration of the SPS. Any thoughts?