Turning Yellow

[lottiece]

Has anyone got and idea why a adult male guppy sliver scales or turning yellow ?  No water tests have shown anything wrong, all reading ok.
 
Tank info..... divided  20 gallon (80 litres) tank which is fully cycled.  3 male adult guppies on one side.  2 female adult guppies and 5 dwarf rasbora the other side.  
 
I will try and get photo later.

 

[fluttermoth]

Could you post the actual numbers from all your tests, please, including pH and hardness, if you know them. 'OK' isn't enough information for us to rule out a water issue.
 
How long have you had the fish, and do you have any idea how old it might be?