Juvenile Green Terror's color fading
 

Recently i bought a juvenile GT from a pet store which was very small with faded color.I kept him in good conditions,feed him small shrimps and now he his active and swims well.But i still doesn't see the color on the tail(orange color).Is there something wrong??
Note- he is alone in a 30gal tank with suitable conditions.I havn't kept him in Oscars tank r8 now.

Dark substrates often produce darker color's on fish.Light colored substrates sometimes cause colors to wash (become lighter).
Juveniles don't present full colors for a few month's

Can i keep him with 6 Oscars tank??They are all about same 4 inch size and my GT is about 2inch.

While the fish would prolly get along,,I would be worried about water quality in your 127 gal tank? as the Oscar's grow to possibly twelve inches each.
These six oscar's will produce a LOT of waste (poop), and 50 to 60% weekly water changes will need to be performed each and every week.
The fish will do poorly with Nitrate level's much over 20 ppm as per nitrate test.