Brown color on yellow Tang

[igadget56]

I have a 125gl 72".
1 yellow tang
1 Powder Blue
1 Tomini
1 Fox Face
2 clowns
The yellow Tang is turning brown
Very slow. A little each day he is not eating.
He hasn't been bullied or fighting. Kind of
looks like it was air brushed on. The day
before I noticed it I did a large water change.
Parameters all check out good. Other fish
having no problem. Oh ya, he just stays in
one place. I did add a GSP and a paly

[Rapide]

They turn brown when stressed. You do have some "aggressive" fish there, maybe they don't directly bully the YT, but it might be the reason...
Your tank is not "too small", but a Acanthurus leucosternon in a tank that size minght get a little chippy... I'm just saying, that it could be bigger... You do know that the Powder blue is an open water fish, right? Also, the PBT can be highly aggressive and quite destructive when it gets in the mood.
Also the tangs from the genus Chtenochaetus can be a bit bossy. I have the Kole tang (C.strigosus) and he isn't even scared of the 3 times bigger Sohal!
And Foxface can also be aggressive sometimes.

Anyhow, it's stress colour, or camouflage. Feed a lot of algae, they tend to be more healthy that way.
And toss the brine away! It is not good food. Feed them mysis instead.
Adult brine shrimp has virtually no nutrition value. You can feed your Protopalythoa with brine...