It is common practice to define pure colors in terms of the wavelengths of light as shown. This works well for spectral colors but it is found that many different combinations of light wavelengths can produce the same perception of color. This progression from left to right is from long wavelength to short wavelength, and from low frequency to high frequency light. The wavelengths are commonly expressed in nanometers (1 nm = 10-9 m). The visible spectrum is roughly from 700 nm (red end) to 400 nm (violet end). The letter I in the sequence above is for indigo - no longer commonly used as a color name. It is included above strictly for the reason of making the sequence easier to say as a mnemonic, like a person's name: Roy G. Biv - a tradition in the discussion of color. The University of Walkato posts a suggested set of characteristic wavelengths to associate with the colors. Wavelengths for ROYGBIV Violet 400nm Indigo 425nm Blue 470nm Green 550nm Yellow 600nm...
Optics for Optometry