There are some varieties of koi that are renouned for their color changing, specifically, the Kumonryu (pronounced Coo-mon-roo) and the Shusui (pronounced Shoe-shwee), but the list goes on and on, and even solid colored koi (or Ogons) can develop small patches of colored spots over time if they're not a real quality fish. Typically, the higher quality of fish you purchase, the less likely it is to do something unexpected with it's color, but this is not always the case. Serious koi hobbiests will often buy a young koi that has not yet fully developed it's color in anticipation of future glory. However, this can be a crap shoot, and that's why a lot of money is spent on koi that have already fully developed their colors.
When a fish starts to change color, it's important to keep an eye on it because bacterial infections may look like a pending color change but actually be a developing sore.
If you have a fish that has changed color, I would love to see it. I will even post it on this page, if you say I can.
Remember, just keep an eye on the progress, and don't give up on the fish as "ugly" until it's had a chance to complete it's metamorphosis!