In them there is nothing else than a certain Power and Disposition to stir up a Sensation of this or that Colour. The tristimulus theory of colour perception seems to imply that any colour can be obtained from a mix of the three primaries, red, green and blue, but although nearly all visible colours can be matched in this way, some cannot. However, if one of the primaries is added to one of these unmatchable colours, it can be matched by a mixture of the other two, and so the colour may be considered to have a negative weighting of that particular primary. The primary Y was chosen so that its colour matching function exactly matches the luminous-efficiency function for the human eye, given by the sum of the three curves in figure 2 . The CIE Chromaticity Diagram showing all visible colours.
Chromaticity depends on dominant wavelength and saturation, and is independent of luminous energy.
Colours with the same chromaticity, but different luminance all map to the same point within this region. Complementary colours, i.e. colours that add to give white, lie on the endpoints of a line through this point. Furthermore, all colours within a triangle may be formed by mixing the colours at the vertices. Mixing colours on the chromaticity diagram. All colours on the line IJ can be obtained by mixing colours I and J, and all colours in the triangle IJK can be obtained by mixing colours I, J and K. 3 Colour Models Colour models provide a standard way to specify a particular colour, by defining a 3D coordinate system, and a subspace that contains all constructible colours within a particular model.