Concept and Utility
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".
Read more about this topic: Imaginary Color
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