One type of imaginary color (also referred to as non-physical or unrealizable color) is a point in a color space that corresponds to combinations of cone cell responses in one eye, that cannot be produced by the eye in normal circumstances seeing any possible light spectrum. Thus, no object can have an imaginary color. But such imaginary colors are useful as mathematical abstractions for defining color spaces.

The spectral sensitivity curve of medium-wavelength ("M") cone cells overlaps those of short-wavelength ("S") and long-wavelength ("L") cone cells. Light of any wavelength that interacts with M cones also interacts with S or L cones, or both, to some extent. Therefore, no wavelength (except perhaps a bit of the far red), and no non-negative spectral power distribution, excites only one sort of cone. If, for example, M cones could be excited alone, this would make the brain see an imaginary color greener than any physically possible green; producing it by seeing light would need some of the red and blue parts of visible light to have negative power, which is impossible.