"A BLACK CAT is bad luck. Envy is green. Passion is purple. A coward is yellow. If you're blue, that's bad. But if you are true blue, that's good. We damn blue laws. We cheer blue ribbons. And we hope that our futures will consist exclusively of red letter days, but red is precisely the wrong color for our financial accounts. In a thousand our idioms we unconsciously reveal the first and most important meaning of color to man: as a talisman beckoning friendly gods and warding off evil spirits. Long before scientists knew that the sun was the source of all color, the Egyptians bestowed on Ra, their sun god, the rank of first king of Egypt. And because purple was the most difficult color for the Egyptians to duplicate, purple became the prerogative of Ra and of all kings since."
from The Magic of Color by Thomas H. Wolf