SUMMARYThe Kubelka–Munk theory, which is commonly applied to ‘pigment mixtures’, adequately predicted the colour of mixtures of synthetic haematite, synthetic goethite and deferrated soil powders. The theory illustrated the higher pigmenting effect of haematite compared with goethite. In mixtures containing haematite the calculated colour coordinates could be combined into simple ‘redness’ indices that were highly correlated with haematite content and were not appreciably influenced by goethite.The theory was also applied to the study of the chromatic characteristics of 98 soils, differing widely in their origin but having an organic matter content<2% in which haematite or goethite was the dominant Fe oxide. The theory showed that the average ‘soil’ haematite and ‘soil’ goethite had colours similar to those of their synthetic counter‐parts, whereas the rest of the soil components could be considered as an essentially ‘grey’ matrix. It was also useful in predicting the haematite and goethite contents of soils either from several reflectance measurements of soil‐white standard mixtures or from the indices of redness developed f