Describing continuous phenomenon like light with discrete increments always requires a little flexibility. For example, at what point on the spectrum does the color change from yellow to green? Different people will place that point in different places. You can see from figure 1 the basic Kryptonian color words, and approximately where they lie on the electromagnetic spectrum.
Kryptonian physiology is able to detect a broader range of the electromagnetic spectrum than that of humans, and the effects of yellow solar radiation extend this ability even further - to the point of producing the "X-Ray Vision" effect. It is thought that the "Heat Vision" capability is also closely related to this increase in visual acuity. Though there has been much speculation, the mechanisms behind these abilities have not been rigorously studied and are not fully understood.
Nuances of Color
In Kryptonian, there are two color prefixes. gAd moves a color towards the red end of the spectrum, while kü moves a color towards the violet end of the spectrum. So, for example, English "yellow-green" would be riz in Kryptonian, while English "yellow" would be Kryptonian gAdriz. As can be expected, this gets fuzzy at the color boundaries—at what point does küIä (orange) become gAdriz (yellow)?
Not pictured above is the color brown: SOtev. This color word does not take the gAd or kü prefixes, but it does take the bright/dark prefixes (see below).
Black, White, and Gray
Black, dol, and white, kriG, each have a prefixing form (leave off the final consonant) that can be applied to any color for "bright" or "dark". For example, red: Iä, can become bright red (or pink): kriIä, or dark red: doIä.