A spectral color is a color that is evoked in a normal human by a single wavelength of light in the visible spectrum, or by a relatively narrow band of wavelengths, also known as monochromatic light. Every wavelength of visible light is perceived as a spectral color, in a continuous spectrum; the colors of sufficiently close wavelengths are indistinguishable. The spectrum is often divided into named colors, though any division is somewhat arbitrary: the spectrum is continuous. Traditional colors in English include: red, orange, yellow, green, blue, and violet. In some other languages the ranges corresponding to color names do not necessarily agree with those in English. The division used by Isaac Newton, in his color wheel, was: red, orange, yellow, green, blue, indigo and violet; a mnemonic for this order is “Roy G. Biv”. In modern divisions of the spectrum, indigo is often omitted. One needs at least trichromatic…