[firemedic1169]

a sub-dominate male will subdue their color to not attract attention, the opposite of a male's color becoming more vibrant when its trying to attract a female

 

[firemedic1169]

Then I would guess stress or a water quality issue.
The fish you took back may be the cause or:
How long has the tank been established?
How often and how much are your water changes?
Have you tested your water lately?

 

[firemedic1169]

I would let the tank settle for a few days now that the bully is gone and see what happens.