The spectrum colours are the colours on the horseshoe-shaped curve on the outside of the diagram. All other colours are not spectral: the bottom straight line is "the line of purples", whilst within the interior of the diagram are unsaturated colors that are various mixtures of a spectral color or a purple color with white, a grayscale color. White is in the central part of the interior of the diagram, since when all colors of light are mixed together, they produce white.]]