The reflectance and transmittance of polymer films containing colloidal particles are characterized by specular and diffuse components which, for a given wavelength of the incident radiation, depend on particle size and composition, particle concentration, and film thickness. Pigment particles do not in general exhibit bulk densities or bulk optical constants. In this paper we introduce a method to determine the mass density and refractive index of non-absorbing or absorbing particles from experimental measurements. A four-flux radiative transfer model is used to describe the optical properties of dilute mixtures of silicon dioxide and titanium dioxide particles in a polymer matrix. The refractive index of the particles is varied from bulk values, in order to achieve good agreement with experimental reflectance and transmittance spectra. The obtained refractive indices can be related to the mass density of the particle material by the Clausius - Mossotti equation.