Genetics has a lot to play with how big the kok and how much the color is affected. But feed good quality food to help. I am currently using NorthFin tropical sticks for my Flowerhorn.
Going further on that note, I'd avoid foods that claim to be made specifically for flowerhorns that increase hump size etc, such as Grand Sumo, XO Humpy Head, Hai Feng etc... a lot of these have "fish meal" and a lot of fillers which just add to the bioload unnecessarily.