The issue with coral lightening usually seems to be nitrogen based moreso than anything. That is unless your phosphates are dangerously low, which is difficult to achieve long term without constantly changing GFO.
You can go about replenishing nitrogen in a variety of ways, dosing amino acids is one. I feel that when people experience corals lightening ans then see improvements after dosing amino acids, it has little to do with the corals actually using the amino acids, and a lot to do with the resulting nitrogen from them breaking down. Keep in mind corals have been shown to be perfectly capable of synthesizing their own amino acids. There is also some doubt on their capability to take in amino acids from the water column.
You can also feed something, most of which will probably be filtered out or cause some other types of pollution in addition to bumping nitrogen a bit.
Or, you can get more to the point and save some money by dosing sodium nitrate or potassium nitrate to keep nitrates up. This method has worked very well for me when I've struggled to feed enough to keep nitrates detectable and certain corals from getting pale. For the longest time I thought it was some type of trace element that was missing since color would always improve after a water change and then start to fade again as the water change became more distant. Once I realized that a water change actually increased my nitrates to being detectable it all made sense. If I dose to keep nitrates detectable I can now go months without a water change and seeing corals lighten (not something I suggest, but I had to tinker). In the past, if I went more than a couple weeks without a water change my corals would more and more pale as time passed and certain corals (montipora in particular) would start to lighten in color.
The above became more apparent after I had lost several fish due to a power outage. Corals were struggling, and upping my dosage on the nitrates helped hugely and meant that I didn't have to feed and increase PO4 unnecessarily at the same time.
