We are describing two methods of making physical colors. First we consider resonant excitation of high dielectric constant nanospheres (for example Si) coupled with absorption of the entrapped radiation. Alternatively we use absorption in metallic spheres, which exhibit negative real parts of the dielectric constant. Peak absorption occurs where the negative real part of the dielectric constant equals twice the positive dielectric constant of the surrounding medium. Absorption can be must larger than what could be achieved in conventional pigments leading to smaller required total volume of pigment.
Manfred R. Kuehnle, Hermann Statz, "Physical Colors By Nanoparticle Resonant Color Pigments For High-Quality Printing Processes" in Proc. IS&T Int'l Conf. on Digital Printing Technologies (NIP17), 2001, pp 239 - 242, https://doi.org/10.2352/ISSN.2169-4451.2001.17.1.art00050_1