Calculated optical absorption spectra of colloidal copper particles in photochromic glasses containing both copper halides and cadmium halides are presented. Comparison with available experimental data supports the previously suggested colloidal darkening mechanism. The dielectric constant of the copper particles is obtained from the bulk one by applying either the mean‐free‐path correction or the quantum size effect correction. It is shown that, using the available experimental data, no conclusion about the superiority of either correction can be drawn.