### Imaginary color

Non-physical, unrealizable, or imaginary colors are points in a color space that correspond to combinations of cone cell responses that cannot be produced by any physical (non-negative) light spectrum.[1] Thus, no object can have an imaginary color, and imaginary colors cannot be seen under normal circumstances. Nevertheless, they are useful as mathematical abstractions for defining color spaces.

The spectral sensitivity curve of medium-wavelength ("M") cone cells overlaps those of both 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, there is no wavelength, and no non-negative spectral power distribution, that excites only M cones without exciting S or L cones at all. The hypothetical excitation of the M cone alone would correspond to an imaginary color greener than any physical green, corresponding to a spectral power distribution with positive power in the green (medium) wavelengths and (non-physical) negative power in the red and blue (long and short) wavelengths.

## Concept and utility

 The CIE 1931 color space chromaticity diagram. The white regions correspond to imaginary colors. The ProPhoto RGB color space uses imaginary green and blue primaries to obtain a larger gamut (space inside the triangle) than would be possible with three real primaries. However, some real colors are still irreproducible.

Real colors are colors that can be produced by a physical light source. Any additive mixture of two real colors is also a real color. When colors are displayed in the CIE 1931 XYZ color space, additive mixture results in a color along the line between the colors being mixed. By mixing any three colors, one can therefore create any color contained in the triangle they describe—this is called the gamut formed by those three colors, which are called primary colors. Any colors outside of this triangle can not be obtained by mixing the chosen primaries.

When defining primaries, the goal is often to leave as many real colors in gamut as possible. Since the region of real colors is not a triangle (see illustration), it is not possible to pick three real colors that span the whole region. It is possible to increase the gamut by selecting more than three real primary colors, but since the region of real colors is not a polygon, there always will be some colors at the edge left out. Therefore, one selects colors outside of the region of real colors as primary colors; in other words, imaginary primary colors. Mathematically, the gamut created in this way contains so-called "imaginary colors".