In modeling color vision, certain visiblewavelengths have special significance. A growing body of scientific work shows that the wavelengths around 450nm, 540nm and 605nm, the so called prime-color (PC) wavelengths, are fundamental to color vision. Perhaps unsurprisingly, these same wavelengths are often discussed in the color imaging literature. Monitors that can display a large gamut of colors and are visually efficient have phosphor-primary peaks at the PC wavelengths. Color cameras that have peak sensitivities at the PC wavelengths have favorable color-balancing properties. Why are the PC wavelengths so important? This paper provides a start toward a mathematical theory to answer this question.