The color and brightness is related to the extent of reflection–diffusion of light on the mineral
surface, which is dependent on grain size, grain shape, and roughness of particles, as well as the chemical composition of the mineral powders (Billmeyer & Saltzman 1981;Bundy & Ishley 1991; Bizi et al. 2003; Ciullo & Robinson 2003; Gamiz et al. 2005).