bicolor blenny

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I have put two bi color blennies together in a 75 gal with tones of hiding places. i noticed the other day the big one picking on the little one. i stopped the fight gave the little one time to get back to his side. i noticed today that the big ones yellow tail is rather black. it is not as yellow like it used to be. anyone know why this might be going on. i can not find any information on it

 

[WXB]

It seems my blenny is almost all black. I found how they will change colors to blend in but it was fine till i added in the little one with in two days of adding him