I wouldn't be so silly as to liken my newly colored hair to the amazing celestial event that is a once-every-75-year occurrence such as halley's comet. I would, however, liken the once-every-75-years thing of halley's comet with the frequency of my desire to color my hair; in that, my itch to go dark happens at a relative certain frequency for me as well. So it's a stretch, but there you have it.