The saturation and the lightness of hematite layers are quantitatively related to the size distribution of the pigments, when applied on skin or a perfect white substrate. The optical properties, reflectance spectra, and colorimetric coordinates are calculated from the complex refractive index of hematite and by use of the radiative transfer equation. Monodisperse pigments are investigated first and the maximum of saturation is calculated as a function of the pigment radius. Polydisperse pigments are then investigated with a log-normal size distribution. The maximum of saturation is then calculated as a function of the width of the pigment distribution, for different mean radii. This modeling can be extended to any mineral pigments.