No, probably just a color change. Comets come in three colors, usually. Brown, orange and white. They may start out several different colors, but they will end up one of the three. Most likely it is simply changing colors due to some environmental factor.
I mostly agree with this, but it could be a Shibunkin, which can stay multi-colored throughtout life. Some of my pond fish have turned a solid color after a while, others stay mulicolored. The development of black or brown along the dorsal fin is a common thing that I see in my fish as they grow up.