While the colors of the rainbow (or spectral colors) form a continuum, people have assigned them to finitely many pigeonholes for centuries and probably millennia. Apparently, Newton used seven colors to describe the rainbow. He wrote in Latin, I think, but we'd call them red, orange, yellow, green, blue, indigo, and violet. Nowadays, schoolchildren are taught the six colors of the rainbow: red, orange, yellow, green, blue, and violet. (I guess the indigo faded in the laundry, or something.) Does Judaism (the Torah) do this: is there some finite number of colors of the rainbow (or spectrum) listed somewhere? If so, is there any (midrashic or similar) significance assigned to that list?
Zohar (Bereishis 18b and in other places) states that the rainbow has three colors, חוור סומק וירוק - white (or pale), red and green.
In Bereishis it associates these three colors with Gavriel, Michael and Raphael. Elsewhere (Bamidbar 215a) it associates them with the three Avos. In one of the maamarim (chassidic discourses) of R. Shneur Zalman of Liadi, he associates them with three types of teshuvah, based on love of Hashem, fear, and "great mercy." Basically, then, all of these relate back to the "three lines" into which the sefiros fall: the right (chessed, kindness); the left (gevurah, severity/strictness); and the middle (tif'eres, mercy, a blend of chessed and gevurah).