Sometimes changes in colors in fish reflects changes in their health. What kind of fish is he? Is the food old, or is he being fed too much? Sometimes minor things like that can lead to color change. Also, when a fish is stressed it might change colors too. Make sure the water temperature is warm enough and that, if he is a betta living in a bowl or similar, the water is getting changed. At least, those are the things that I looked at when my betta changed. There could be other reasons, I`m sure. But whatever the reason may be, that must have been an interesting surprise!