A surface of a given chromaticity is seen as more colourful, the more luminous it is; if the surface is black, its color is seen more vivid the less luminous it is. In the trichromatic case, the spectral reflectance of a most luminous surface is binary and may have at most 2 transitions
or discontinuities, in the wavelength domain. Assuming a spectrally flat illuminant, we give analog conditions for the spectral reflectance of a surface to be most luminous, when observed through photoreceptors having four types of spectral function.