Yellow Watchman Goby color change

My wife sent me a pic of our YWG the other night when I was away at work. She got home late (1am) and turned the light on to check on the fish and she saw his color was grey/faded.
The next morning she checked on him again and he looked fine! (See pics)

Was something wrong with him or is it normal for his color to fade at night time?

Thanks!

Normal. They are trying to hide from predators.

Mine hasn't been yellow in months. He was when I bought him, stayed that way for a week or so then he turned gray.