Hi PapaBear,
I did fed them with colour enhancing food with a Japanese brand. The food indeed help the hi up and fulfill the scale. But I didn't count the side effect of the food since I use automatic feeder machine to feed the kois. I mixed the colour enhancer food with the wheatgerm type food and then pour it to the machine and set 8 feeding times per day.
Probably the colour enhancer food could cause the yellowish shiro, but I just confuse that her sibling doesn't get affected at all.