Black for Capricorn, I read once (for a dark, cold time of year).
Capricorn is proably one of the dimmest constellations, but around 2500BC, it was important because the first day of Spring occured when the sun was in the constellation. So it's regarded as one of the oldest named astrological figures.
Capricorn was named for Pan, a goat that tried to turn into a fish, but only half-completed the task. But it screamed, and saved the day. (I forgot why, and my copy of Bullfinche's Mythology isn't with me right now.)