There are many illnesses that do not effect appetite and some that even increase it.
I am not a koi keeper but try googling "color changes in koi" or "color loss in koi", this should steer you towards further information.
so i didnt think koi changing colors was a good thing from the start.
lost this koi last night
he was like a year old.
no clue what happened.all other fish/koi are fine.
don't worry it's normal. I had one years ago that was white with an orange spot on the tail area. One day I looked in the tank and it was pure white. no known reason, no sign of illness. It lived for at least several years afterwards then I had to get rid of my fish due to a long distance move.
damn old thread, but anyways, that is a low quality Showa. showa and other koi like Kumonryu will change colors all the time. there are countless articles about getting a proper showa to stabilize as it matures, its like a science or religion.
nothing is wrong with the fish, its just not a good one. as for its death, you can think what you may but it had nothing to do with its change in color