Hi Robin, to predict the colour (chromaticy) of a source requires convolution of its spectrum with the colour receptor profiles of the eye (or equivalent standardised profiles, such as those of the CIE); two convolutions are required, the third can be found via normalisation, x + y + z = 1. This allows a plot of x and y, say, on a two dimensional ‘colour triangle’, with ‘red’, ‘green’ and ‘blue’ at the ‘corners’. A blackbody has a locus of x, y values running across the ‘triangle’. The method then boils down to matching the x and y values of the light source with those of a blackbody at a given temperature (as closely as the values allow). Chromaticy can also be measured using a chroma meter. Hope that helps.