I have a few more questions:
How long have your parameters been stable at these levels?
How long have you been losing colour? (Fast change, gradual change?)
Were the corals browned when you got them or normal colors?
I might go against the grain here and suggest that your water is lacking in nutrients. Zooxanthellae require nutrients to survive, in addition to proper light - like almost any photosynthetic organism.
From reading, I'm guessing that your zoox are "starving" due to the light nutrient load. At balanced levels the zoox provide colour. Too much light or too little and they tend to bleach. Too high of nutrients and they tend to brown, too few nutrients and they tend to wash out - like you're seeing.
Since you're seeing reasonable growth, you can assume the coral is healthy for now; the zoox isn't able to form population densities rapid enough to show colour. This limits UV
protection to the coral itself though and could lead to a bleaching event if you tinker with the lights.
Slowly add livestock or cut back on your carbon dose until you can sustain .25 or .5 ppm
and maintain around .03 ppm
. Perhaps an amino acid supplement for corals could help.
Another though could be to slightly lower the calcium & Alk
levels to attempt to decrease coral growth - reducing the hosts energy consumption. But I would think you would rather maintain growth levels.