The optical characteristics ͑absorption and reflection coefficients͒ of metal-dielectric mixtures, as well as of nanoshells with such mixture as a shell layer, are considered within the framework of the effective medium approximation. The evolution of plasmon characteristics and optical properties with changes of metal composition in a shell layer and its thickness is studied. The results are applied to the problem of enhancing the luminescence characteristics of semiconductor nanocrystals covered by metallic nanoshells.