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, (formula available in paper) and 3) the reversible coloration and decoloration for an Ag single-doped glass.