Angel fish fading in colour

[Twwt]

One of my colin angelfish is getting pale in colour, not yellow by it pale grey, does anyone have the same experience as well? How to make its colour back?
It’s not the weakest, middle in size and the smallest one is brighter in colour instead

 

[LiLinka]

What are you feeding how often?

 

[Twwt]

What are you feeding how often?

I feed them with wide range of food. Frozen, flake and pellets.
working days only once per day. It’s in normal lemon yellow colour before, just wonder if fishes pairing up will cause the female changing its colour. The dominant one and the smallest one are normal but the one in the middle fading in colour