Hiya petalop and welcome to Aquariumadvice
Clown loaches change color like I change undies *grin*
If your parameters are good, and there is no change in behavior, its likely your clown is simply displaying his mood. My clowns will "gray out" when they are fighting over food. Its part of the dominance play. Do read the article here from loaches.com and see if it fits your situation: http://www.loaches.com/markings.html
aka Cycling Guru and the Ich Slayer
*glares at Terry and QTOFFER*
Card carrying member of FTAS & GCAS.