You can think of comet as "dirty snowballs" that come from the outer regions of the solar system in highly eliptical orbits. Many only come once; others (like Halley's) return after many decades. As they approach the sun, some of the ice, other volatile chemicals and dust vaporize and are blown away by solar radiation, leaving a luminous "tail" of reflected sunlight and fluorescence. The blue color in the spectrum comes from C2 gas in the tail, but the eye sees the comet mostly the color white.
