Know that between the apex of the rainbow and the circular area of the solar halo (mentioned earlier in our treatise) there is a certain equivalence. They have the same cause, which is the impact of sunshine on particles of humid vapor that are present, and its reflection back in the direction of the sun.
The visible hues are four. These correspond to the four qualities which are heat, humidity, cold and dryness; and also to the elements which are fire, air, water and earth; and also to the seasons which are summer, autumn, winter and spring. And also to the four humors - which are the black and yellow bile, the phlegm, and blood - do they bear a similarity. To the colors of the flowers of plants and trees they bear a formal analogy, for when the seven colors of the rainbow come out it is a sign of the air's humidity, the proliferation of rain, and the increase of the grasses, orchard fruits and grain crops. Its appearance is as a joyful proclamation which nature presents to animals and humans, announcing the fecundity of the season.
As for those vulgar interpretations of the rainbow which read indications of the coming year into the relative intensity of its colors - with predominance of red for the spilling of blood, yellow for victims of illness, blue for war and green for fertility - these are altogether a matter for omen-readers.
From his treatise On the Cosmos