Spores exhibit changes in colour with increasing depth of burial; these changes have found wide application for thermal maturity determination in hydrocarbon source rocks. At present colours are determined visually. Here the application of the Commission Internationale de l’Eclairage colour system to the measurement of spores is described such that colour can be defined by the chromaticity co-ordinates (x, y) and the luminance (L) which define a colour solid. The colour changes seen in spores with increasing burial define a maturation pathway in this colour solid which shows two distinct trends. The first is a well-defined progressive colour change to a cusp at which a broad spread of colours from yellow to brown occur in a single sample. This is the interval over which hydrocarbons are generated and which is clearly defined by these colour differences. The second trend is through a series of brown colours with an increasing lack of sensitivity of the spores to colour changes. These changes are calibrated to vitrinite reflectivity. The data and conclusions are amplified and supported through changes in colour, chemistry and exine reflectivity in artificially matured Lycopodium spores. Comparisons are made with existing qualitative spore colour scales. The implications for maturation determination are that spore colour is a good thermal maturity index for the interval prior to oil generation and that it is suitable for quantitative modelling studies. It demonstrates the interval of oil generation at the cusp but after this point it becomes inaccurate and inferior to other methods such as vitrinite reflectivity.