Color-changes in our fish is quite common and may be caused if the fish is kept indoors away from the natural sun light ... and may be suffering from pigment loss in the skin. The Kohaku varieties are well known for losing their colour - and if it is a Tancho this this will be unfortunately, the red Hi spot. 

But again stress to a particular fish can sometimes result in the fish throwing out every bit of colour or pattern it has .. there is nothing that you can do to prevent this and you will, unfortunately end up with a 'Shiro Muji' in the case of a Koi .. which will then show as an 'Off White' fish ..  but it will still be in good health and still have that same friendly nature ... nothing else changes .. and sometimes it can be a great improvement to the appearance of the fish if in its previous state it did not have a good colour, or pattern formation ... So not all is bad ... 

But as it usually happens - sod's law it is called, it may well be the case that this colour loss occurs in your best Koi .. That happened to a very close colleague of mine about 5 years ago ..  Not really being a Koi collector that would normally transport his prized koi to the shows, on this particular case he did and the Kohaku in question actually was the Show Champion that year ... Within 3 months that beautiful Kohaku began to loose its colour and within 6 weeks after that was swimming around in my own big pond as a Shiro Muji.... and of course there was nothing that either I or my colleague could do to prevent this happening ...