Any of the shades of red, green, or blue that can be mixed to form white and to match all other spectral colours and also certain non-spectral colours such as purple. Primary red, green, and blue are often defined as light with wavelengths of 670, 535, and 470 nanometres respectively, but any three shades may be used, provided that none is equivalent to a combination of the other two. Also called a primary colour, though that term is ambiguous. See also CIE colour system, chromaticity, colour equation, Grassmann's laws (perception), primary colour (1), Rayleigh equation, tristimulus values. Compare psychological primary, secondary colour, subtractive primary.