To reveal the light-color modulation mechanism of firefly (beetle) bioluminescence, we investigated the spectroscopic properties of the phenolate anion 1-O- generated from 5,5-dimethyloxyluciferin (1-OH) using various base/solvent combinations. Phenolate anion 1-O- is a model compound for the keto form of wild-type oxyluciferin phenolate anion (OL-), which is postulated to be the emitter of the bioluminescence. The fluorescence maxima of 1-O- were found to depend on the base/solvent combination used, and they varied in the range 541-640 nm, which covers the almost whole range of the bioluminescence emission maximum. In a polar solvent, where (1)(1-O-)* and the countercation (the conjugate acid of a base) make a solvent-separated ion pair or a free ion couple, the emission maxima of 1-O- were found to be modulated by the solvent polarity. In a less polar solvent, where (1)(1-O-)* and the countercation are formed as a contact ion pair, the strength of the covalent character of the O8'center dot center dot center dot H bond between (1)(1-O-)* and the countercation is operative. The effect of the base/solvent combination on the emission properties of (1)(1-O-)* was also verified using fluorescence lifetime measurements and density functional theory calculations on 1-O- and its ion-pair models. On the basis of these results, we propose the following light-color modulation mechanism: (1) the light emitter is the excited singlet state of OL- [(1)(OL-)*], and (2) light emission from (1)(OL-)* is modulated by the polarity of the active-site environment of a luciferase and the degree of covalent character of the O8'center dot center dot center dot H bond between (1)(OL-)* and a protonated basic moiety in the active site. Mechanisms for variation of the bioluminescence colors and their applications are discussed.