Imaginary Colours

They are colours that don't exist physically.

For example, in LMS Colour Space, there are overlaps for wavelengths between L, M ans S cone cells. So some combinations like non-zero M values and zero L and S values are impossible.

e.g. If we could have a colour that excites only M cone cells, then we'd see a colour greener than most greens.

Also, in a normal trichromatic additive colour space like RGB Colour Spaces, pure spectral colours would imply negative values for at least one of the three primaries.