I have 2 butterfly Koi that I purchased last year. They are yellow in colour and now are begining to have orange spots on them. Is this normal or are they infected by something?
How big are the koi? When the koi are young their colors can change.
Slook, It is not at all uncommon for yellow fish to develop orange spots on them. As long as the spots are orange and not red I don't think there is a problem. But keep an eye on them and if the spots start to look red than take a closer look.
It is VERY common for a Yamabuki to develop orage spots...black spots can also pop up as well.