Hello, welcome to the forum.
Koi is marble based - meaning its color or pattern may change. So it's normal for your koi to change to red.
Dull can be caused by the type of red (genetically). It can also be caused by less than ideal water. I'm sorry, but your boy doesn't look 100% healthy. His fins suggest he is either recovering from a disease or is unhealthy (compare pictures before and after).
Koi is marble based - meaning its color or pattern may change. So it's normal for your koi to change to red.
Dull can be caused by the type of red (genetically). It can also be caused by less than ideal water. I'm sorry, but your boy doesn't look 100% healthy. His fins suggest he is either recovering from a disease or is unhealthy (compare pictures before and after).