The Usambara effect, i.e. a change of tourmaline colour from deep-green to dark-red with increasing path length of light, has been studied by optical absorption spectroscopy and colourimetric calculations on a sample of Tanzanian tourmaline of predominant dravite composition with 0.12 apfu Cr. For comparison a dark-green vanadium-bearing tourmaline from Tanzania (0.05 apfu V), which does not show such an effect, was also investigated. As established, the Usambara effect, by its nature, is closely related to the alexandrite effect, although in this case the colour change is not caused by change of spectral composition of the light of illumination, but by spectral positions of the spin-allowed absorption bands of Cr3+, a specific ratio of light transmission in two windows of transparency, green and red, and by non-linear, exponential dependence of the light transmittance on the thickness of sample. A threshold chromium content must be exceeded for the Usambara effect to show, that is, sufficient chromium for there to be two deep and well-demarcated windows of transparency in the visible range. The overall colouration results from mixing of two additive colours coming through the windows of transparency. A dark-green chromium-bearing tourmaline from the Ural Mountains (0.40 and 0.20 apfu Cr and Fe, respectively) shows how admixtures of other chromophore ions, namely, Fe2+ and Fe3+, can suppress the Usambara effect in tourmaline.