A colour can be described as a mixture of three other colours or "Tristimuli". Typically RGB for CRT based systems (TV, computer) or XYZ (fundamental measurements). The amounts of each stimulus define the colour. However, it is frequently useful to separate the colour definition into "luminance" and "chromaticity". Lower case is always used to signify chromaticity coordinates, upper case always signifies tristimulus values (or amounts of the primaries). Chromaticity coordinates can be plotted on a two-dimensional diagram that defines all the visible colours, luminance is normal to that diagram.
CIE XYZ (1931)
CIE YUV (1960)
CIE YU'V'
CIELUV
CIELAB
CIE Colour Difference Formula.
CMC colour difference formula.