You posted a couple of things I found confusing. You report no measurable organics in your tanks, but your water exchange and feeding schedules makes that look pretty near impossible to me. If excess food is going in the sump and you are filter feeding, with the skimmer not making any skimmate, doesn't added up to me very well. Where are the organics going? i would double check those phosphate and nitrate numbers.
corals look brown when in higher organic laden systems with not enough light. They color up as organics go down and lighten up when lighting levels go up and the excess algae stops growing in their flesh.
I wouldn't dirty up the water IMO
. In some systems with certain types of filters, or systems that carbon dose, there can be an imbalance between nitrates and phosphates. In those cases some will add nitrates to help bacterial growth when nitrogen becomes limited. It takes 16 times more nitrate to build a cell than it does phosphates, so when nitrates go to zero, you need another mechanism to bind phosphates, like GFO
or phosguard. But I doubt that's your issue.