His complete change in color will take some time. He is already showing a ton of potential! The fact that he is biting you already shows he has the typical "flower horn responsive/aggressive attitude" which I always feel is a good sign. Please keep us updated.

it's because they have build their territory and yours probably wont accept any other fish in his tank just like my red devil. it's hard for me just to wipe off the algae on my weekly water changes.