In my experience, excess nutrients can cause color loss/browning of corals; in this case, I feel GFO can be a valuable tool to help get the P down to a range 0.02-0.03. But if you can't measure your P, then GFO actually may make matters worse.
If you want to address your algae issue, IME, GFO will not help given your situation, since the algae is already established its foothold.
I would try to ID the algae, and determine if you can eliminate it via AlgaeFix or purchase a fish/clean up crew that specialize in consuming your algae type as a first step (there's a huge thread on RC). I had a similar situation to you relative to algae. (no issues with lack of SPS color, however). I had very low levels of N and P, but could not get rid of the green HA, in my system, even with the use of GFO. If your nitrate and phosphate readings are accurate, then IMO the lack of color is likely due to the ULN. GFO will only drive the nutrients lower, but it will likely not eliminate the algae, and it may even make your goal of getting more color in your corals worse.
In my case, I dosed AlgaeFix marine for a month, and essentially eliminated the nuisance algae from my system. While I dosed the AF, I ran my skimmer wetter than normal and used GFO to reduce the P during the algae die off, and did a 5% water change weekly.
Once you have the algae under control, feed your fish in your system frequently, and they should produce enough "poop" to feed your corals, and that will bring out the color in conjunction with good lighting, proper flow, good skimmer and good husbandry practice. If you read some of the threads on ULNS, they all do something to feed their corals to bring out the corals. The use of GFO once you have the algae under control will help to keep the algae at bay and your P in a range that promotes growth and coloration.
Good luck!
