You may often notice when keeping koi that the colours and patterns they show when you buy them are not the same as after a year or more. This happens for a variety of reasons and today we will go through the main reasons, and whether you should be concerned.
The first, and main reason is simply genetics. Each fish has a combination of dominant and recessive genes from their parents that determines the colouration and markings that it will show as it grows. For example, a Kohaku will have genes corresponding to a shiroji (white) base colour and hi (red) markings while a Showa will have genes corresponding to sumi (black) base and hi and shiroji markings (red and white, respectively).