Just wanted to add, if your not already feeding pellets, it would be a good idea to start feeding them as they are a lot less messier then flakes and your water quality will improve.
what a great pic of a fishes gut. It shows how the food just travels straight through and that fish dont have stomaches. It may be the fish is eating all the green and blue flakes if there are any.
I would change to a sinking granular feed if you can get it.
Thats just pigment and a lot of shubunkin fish have it. Its nothing to worry about though and is really common, your fish may well get some calico markings as he gets bigger.