models capable of representing spectral colors, such as CIELUV, a spectral color has the maximal saturation. In Helmholtz coordinates, this is described as 100% purity. In dichromatic color spaces In dichromatic color vision there is no distinction between spectral and non-spectral colors. Their entire gamut can be represented by spectral colors. Spectral color terms The spectrum is often divided into color terms or names, but aligning boundaries between color terms to a specific wavelength is very subjective. The first person to decompose white light and name the spectral colors was Isaac Newton. Early in the study of radiometry, Newton was not able to measure the wavelength of the light, but his experiments were repeated contemporarily to estimate wavelengths where his color term boundaries probably lay. Newton's color terms included red, orange, yellow, green, blue, indigo, and violet; this color sequence is still used to describe spectral colors colloquially and a mnemonic for it is commonly known as "Roy G. Biv". In modern divisions of the spectrum, indigo is often omitted and a blue-green color is sometimes included. Some have argued that Newton's indigo would be equivalent to our modern blue, and his blue equivalent to our blue-green. However, his nonintuitive choices can be better explained. In the table below, note how wavelength is not proportional to hue (which is approximately perceptually uniform). Color systems such as ISCC-NBS attempt to divide the spectrum into sections that appear perceptually uniform. On the other hand, Newton's sections are approximately uniform in size as they would have physically appeared in the diffracted spectrum, i.e. each about 40nm "wide". In this theory, the sections were divided without influence of his own perception, and each section was then given a name that best suited its average color. In contrast, the sections in the ISCC-NBS spectrum vary greatly in wavelength range, but are more consistent in the hue degree range. Both instances deviate from the basic color terms used in English, only some of which are spectral colors. The table below includes several definitions where the spectral colors have been categorized in color terms. The hue that a given monochromatic light evokes is approximated at the right side of the table. Extra-spectral colors Among some of the colors that are spectral colors are: Grayscale (achromatic) colors, such as white, gray, and black. Any color obtained by mixing a gray-scale color and another color (either spectral or not),