SynopsisThe mechanism of colour change during the pigment dispersion process and during powder application to the skin surface was studied by examining the colour extension properties ofYellow. Red, andBlackiron oxides in white powder bases. A new measure for the evaluation of the degree of colour extension was devised. This measure corresponded well with human visual sensation, and was found to be useful for comparing the rates of colour extension of pigments. The rate of colour extension of pigments was found to vary in the order:YellowacicularRedgranularRedBlack.This result explains colour darkening and the change towards reddish tones during the pigment dispersion process, becauseYellowextends fully in the early stage of the process butRed, especially in granular form, andBlackcontinue to extend even in the later stage. AcicularRediron oxide is more favourably employed than the granular form to attain sufficient colour extension of foundations with lower energy expenditure, and to decrease the discrepancy between the cake colour and the colour of applied powder.Le développement des oxydes de fer et effet sur les nuances des fonds de teint compact