As per above, Id look at water chemistry first and nutrient levels. Typically browning is due to either high nutrient pools, or it can be a sign of worn out lamps or falling water clarity (take some water and put it into a white container and look down through it to see if it has a yellow tinge).
If it is just yellowing water, then carbon will sort it out, although I'd advise caution and only use a small amount (about 1/4 a cup per 200 gallons) to start with, run for just a few hours per day over a week or so. If you clear the water too quickly you can shock the corals by suddenly increasing the amount of light thats hitting them, or simply by mking rapid changes in the water chemistry and reducing organic pollutants.
The only other times i've seen loss of flourescent green pigmentation is when lamps or actinics have gone well past thier replacement date and have shifted out of range, or simply lost much of the intensity. Usually a new set will ring everything back again, but as above..take it easy and reduce the light period for a while, graually increasing it back to normal durationover a few weeks to allow the corals time to adapt to the higher intensity.
As a side note. if you have a mix of soft and hard corals, then its possible that over time, the softies have grown to a stage where the toxins they release are starting to affect the system as a whole, in which case it can be time to thin them out to a more managable size that doesnt affect water quality so much.
Regards
Simon.