It is shown that reduction of metal salts in aqueous, optically transparent host dispersions of inorganic (imogolite) fibers provides a general and convenient preparation technique for stable dispersions of metallic nanoparticles. Stable hydrosols are obtained for gold and silver particles and various bimetallic colloids (Au/Ag, Au/Pt, and Ag/Pt). Colloidal stability is achieved by adsorption of particles on fibers; no stabilizing polymer is required. The optical properties of the obtained colloids are studied and compared with theoretical predictions according to Mie theory. This shows that the bimetallic particles consist of layers of the metals, rather than a mixture at the atomic level, except in the case of Au/Ag.