According to equal and incremental steps in the appearance of colours.
Non-spectral colours
Non-spectral colours are produced by additive mixtures of wavelengths of light.
Examples of non-spectral colours produced by two spectral colours are:
Purple – produced by mixing wavelengths corresponding with red and violet. Red (740nm) and violet (400nm) are at the extreme limits of the visible spectrum.
Magenta – produced by mixing red (660nm) and blue (490nm).
Mauve – produced by mixing orange (600nm) and blue (450nm).
Examples of non-spectral colours produced by three spectral colours are:
Tints
Greys
Shades
So all achromatic colours are non-spectral colours.
Whilst both spectral and non-spectral colours are produced by mixing a combination of colours corresponding with different wavelengths of light:
The RGB colour model produces a full gamut of colours by mixing red, green and blue primary colours in different proportions.
The CMY colour model produces a full gamut of colours by mixing cyan, magenta and yellow primary colours in different proportions.