In 1789, when neither the physical basis of hue nor the retinal basis of color perception was established, the mathematician Gaspard Monte stated firmly that our color perceptions do not depend on the absolute value of the physical variable, but are influenced by the context and in particular by our estimate of the illuminant. He used this insight to explain color contrast effects and the Paradox of Monge (the desaturation of red objects seen through a red filter). He proposed that we can estimate the chromaticity of the illuminant in any scene because all surfaces reflect to us varying mixtures of (i) the body color and (ii) a specular component that represents the illuminant. He also realized that white objects have a special property: Provided that they are illuminated by a single illuminant, such objects exhibit no variation in chromaticity across their surface. Thus at least one of the unique hues exists as an external reference on which observers can agree. It is suggested that other unique hues may also have a basis in the external world.