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We have been developing a technique of coloration and decoloration of glasses by photo-irradiation and heat-treatment for application to easily recyclable colored glass products. The mechanisms of the photo-induced coloration of glasses used in this research are a) the photo-induced defects (color centers) formation, b) the photo-induced change in oxidation state of ions, and c) the photo-induced formation of nanoparticles in glasses. The subjects for application of these phenomena to recyclable colored-glass products are presented. The research examples for each mechanism are presented in this paper as follows; 1) the effect of the doped Fe ions on the optical density and stability of coloration due to color centers, 2) the coloration by the change in oxidation state, 2+ > 3+ and 3) the reversible coloration and decoloration for an Ag single-doped glass.