Very good representation of a gill
. Try using the same paint as used on the sides with a slightly darker tint for the stripes. After the stripes are done blend them together using the same color across the backbone. Follow that with your brown color over the backbone and put some spots of various sizes in the stripes using the brown or some other color that looks "gill". You have a good style of your own, go with it. Experiment around with texture and color in ways that look "right " to you.