We report a new generic method of reversibly controlling the photochromism of spiropyrans. It was found that the photochromic effect of spiropyrans can be reversibly switched on and off by addition and removal of carbon dioxide (CO₂) to spiropyran in alcohol solutions containing an amidine (i.e., DBU) that acts as a CO₂ sensitizer. Spiropyrans are not photochromic in the presence of DBU but photochromic when CO₂ is subsequently added to the solution. The CO₂ is readily removed by inert gas bubbling, thus allowing facile activation and deactivation of the photochromic effect. Carbon dioxide, without the presence of the sensitizing amidine, had no effect on photochromism of the spiropyrans. Other photochromic dyes classes such as spirooxazines and chromenes are not affected by this CO₂/DBU stimulus. As a result, orthogonal activation of mixtures of spirooxazines and spiropyrans was achieved to provide four color states (clear, yellow, green, and blue) by varying the combinations of the stimuli of UV, visible light, CO₂, and CO₂ depleted. This finding now permits the many applications using spiropyrans to be CO₂ responsive.