Foxfaces are known to change their colour. For instance, my Magnificent foxface will change colour if someone startles him by walking up to the tank fast. Foxfaces are generally shy and it takes a while for them to become acclimated and comfortable in your tank. I have had mine for 2ish months and he still can be skittish.
Did you quarantine him first? If so, for how long?
At this point, I wouldn't be too worried (at least if you have quarantined him), he could be just adjusting his colour from the light coming on. Usually at night (aka when the lights are off)mine will turn into a camo black and grey and will readjust after lights have been on for a bit.