Imaginary colors
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The CIE 1931 color space chromaticity diagram. The white regions outside the black line correspond to imaginary colors. (The colors in this figure do not reproduce the actual colors in the diagram, due to the limitations of RGB computer displays.)

The ProPhoto RGB color space uses fictitious green and blue primaries to obtain a broader gamut (space inside the triangle) than would be possible with three realistic primaries. However, some realistic colors still cannot be rendered using the colorants that are available.
A fictitious color or imaginary 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.[1] No physical object can have an imaginary color.

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 and no 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. Such a "hyper-green" color would be in the CIE 1931 color space chromaticity diagram in the blank area above the colored area and between the y-axis and the line x+y=1.

Imaginary colors in color spaces
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Further information: Color space
Although they cannot be seen, imaginary colors are often found in the mathematical descriptions that define color spaces.[2]

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 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 cannot 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. The gamut can be increased by selecting more than three real primary colors, but since the region of real colors is bounded by a smooth curve, there will always be some colors near its edges that are left out. For this reason, primary colors are often chosen that are outside of the region of real colors—that is, imaginary or fictitious primary colors—in order to capture the greatest area of real colors.

In computer and television screen color displays, the corners of the gamut triangle are defined by commercially available phosphors chosen to be as near as possible to pure red, green, and blue, within the area of real colors. Because of this, these displays inevitably exhibit colors nearest to real colors lying within its gamut triangle, rather than exact matches to real colors that plot outside of it. The specific gamuts available to commercial display devices vary by manufacturer and model and are often defined as part of international standards—for example, the gamut of chromaticities defined by sRGB color space was developed into a standard (IEC 61966-2-1:1999 [3]) by the International Electrotechnical Commission.