How are you measuring phosphates and nitrates? What is your tempand your salinity(specific gravity)?
If the corals are losing their color, are they browning out? getting pale looking? or white? If they are browning, it is a phosphate/nutrient issue or lighting issue. It could also be water parameter issue, but you would be seeing tissue loss I would think, versus seeing a change in coloration. Do you run a refugium? Without running a skimmer, what are you using to remove proteins, wastes, and nutrients? Also, how often do you do water changes, and have you switched salt mixes recently or skipped a couple water changes?
Zooxanthellae is an algae( a dinoflagellate). It's color is brown and/or green(drab green). When there is an abundance of nutrients in the form of nitrates, nitrogen, or phosphates, then the algae blooms in the corals. It does not allow the natural pigments of the coral to show, you are seeing the zooxanthellate. Plants also like a lower kelvin temp of lighting. As our bulbs degrade over time, the spectrum shifts. This shifting in spectrum can also cause algae populations to grow more.
If you only have a color shift, but everything looks healthy still, then I would do a 25% water change. Then in 3 days, do another 25% water change. After another week, do another 25%, and then do regular weekly 10% to 15% water changes. Without running a skimmer, or ozone, or something to remove DOC(dissolved organic compounds), then you have to do large water changes to remove them, and dilute them. If you have a refugium, make sure you harvest the algae on a regular basis to remove thos nutrients fromt eh system. SPS, acropora and montipora, are very sensitive corals. They shift in color easily. The most important thing for them is stability.
The most difficult thing for us is to keep our tanks clean. They are nothing like the ocean. We are polluting them daily with feeding the fish, and minor and major elements are getting used by corals and not getting replaced. A small water change just dilutes the pollution, and it continues to build over time. The key is trying to keep the water as clean as possible while feeding the fish and corals. This means strong water circulation to keep food and detritus in suspension until it can be filtered out and removed. Even with strong water circulation, there are always some dead spots where detritus will settle out or get caught. This then basically sits and rots and pollutes the tank further. Find these spots and make sure they are cleaned when you do water changes. Hopefully we can help you get those colors back.