Color associations to the signs and planets are not rigidly assigned, and are based on subjective psychological affinities. Some are no-brainers, like red being associated with the planet Mars and the sign Aries (which is “ruled” by Mars) -- the war god evokes blood, anger (seeing red), the red planet, even the iron (oxide) of weaponry, which is also the source of blood’s color. Then the Moon and its associated sign, Cancer the Crab, suggests silver or white, also the moonlike pearl and nacre, esthetic essence of the crustacean.
Some signs have less definite color associations, but sun-ruled Leo, the Lion, one of the 3 firesigns, is obviously related to gold, yellow, orange, and to sunlight itself, if that can be called a color. Aquarius is the sign directly opposite Leo in the zodiac and the two signs are poles of a larger system. Hence, as Leo is sunlight, Aquarius is the spectrum of frequencies of which sunlight is composed, i. e. the rainbow. As Leo speaks of the Sun complacently imagining itself the center of its universe, Aquarius reminds that each of the infinite stars is itself a sun. (And each star has its unique spectral analysis, or rainbow variant.)
This metaphor generates a whole rainbow of antinomies. Leo is the autocrat, Aquarius the anti-authoritarian rebel, democrat or collectivist; Leo the individual, Aquarius the species; Leo the Self, Aquarius the group; Leo egoistic subjectivity, Aquarius detached, scientific objectivity. Leo is the heart/sun/nucleus, Aquarius the circulatory system/orbiting planets/electronic current. Aquarius is a political sign, the rainbow a joyous symbol of the coalition of minorities, yet Aquarius can also be conservative in characteristic ways, embracing a libertarianism in revolt against an oppressive consensus.
Altho an air sign, Aquarius, in ruling circulation, is associated with rivers and streams (whence the flowing symbol of the waterbearer), and more abstractly, with the cycle of ocean, vapors, winds, clouds, rain, river, ocean. The rainbow (an unexpected element of that cycle) is associated with promise and hope, and Aquarius is associated with futurity: the first 10 signs cover the known past and present, Aquarius, the 11th, the airy, insubstantial future, or a cuclical concept of time and eternity, the Eternal Now, “modernism”, the epiphanic Instant, the electrical zap, the accident, the gratuitous, the lightning strike, etc.) With respect to the future, both utopia (for instance the neotribalism of “Age of Aquarius” hippiedom) and super-rational distopia are offered. Aquarius is thus particularly associated with both the threat and promise of ever-increasing scientific objectivity, the very concept a transcendent abstraction called Knowledge, Truth or Mind.
" . . . R is Rubretta and A is Arancia, Y is for Yilla and N for greeneriN. B is Boyblue with odalisque O while W waters the fleurettes of novembrance. . . . Winnie, Olive and Beatrice, Nelly and Ida, Amy and Rue. Here they come, all the gay pack . . ." The 7 Rainbow Girls, a cavorting daisy-chain of colored scarves and flowers, in Finnegans Wake (Ch. 9) by James Joyce (b. 2 Feb. 1882)