I have a royal nile perch myself. It has patches of brown and gold on the body compared to the olive/gray/white coloration of the nile perch. Most sources say the royal nile perch gets up to 2' but I dont know if this is true myself. Mine is around 14" and the brown and gold coloration is still very much apparent on the fish.
Would love to see yours. I love nile perch, I remember as a kid I saw a perfect spcimen at 18-20" or so from memory, black with white marble, it was incredible. I've also raised some nigerian varieties before that were uniformly brown at 12" and beyond. I still believe its different. The regular lates niloticus grows very fast, the royal marking ones grow much slower and seem to stay smaller.