1 fish has faded, but active & eating well. Will colors return?

[Kurious]

Have you tested your water?

Yes

If I did not test my water...

...I recognize that I will likely be asked to do a test, and that water tests are critical for solving freshwater health problems.

Do you do water changes?

Yes

If I do not change my water...

...I recognize that I will likely be recommended to do a water change, and water changes are critical for preventing future freshwater health problems.

I have a 40 gal tank with 1, 2.5" Ryukin goldfish. Had a quarantine tank running for new Ranchu buddy. After 2 weeks, I put the mostly black calico ranchu into the main tack. Tank water temp, parameters and pH are identical, but the Ranchu's black coloring turned brown after about a day. That was 4 days ago. Otherwise he is still very active/curious and water is tested daily. I know stress and disease are the main causes for fading, but other than being moved and a new tank mate, I'm not sure what else to do to de-stress him. BTW, the faded Ranchu is more active than the Ryukin, so it's not like his tank mate is bullying him, etc.

Is it just a matter of waiting it out? The fish didn't fade at all in the 2 weeks of quarantine. Is it possible his original colors will come back in full soon...or ever?