Although the recording system of the leuco dye and phenolic color developer has been employed in practical applications, the basis of colorless-to-coloring thermochromism had not been investigated, which prevented us from controlling the system. We have succeeded in isolating the six colored forms of a leuco dye colored by phenolic color developers and clarified the molecular structures by X-ray crystallography for the first time. It has been revealed that a leuco dye and phenolic developers combined with each other by the hydrogen bond between carboxylate moiety of a leuco dye and OH groups of developers. Correlation with decolorable toner properties and colored-form structures has been revealed; coloring property increases with an increase in the number of hydrogen bonds in colored forms, and erasing property increases with elongating hydrogen bond distance between the leuco dye and developers. Also, a feature of colored forms for toner with both high coloring and erasing properties has been suggested; expansion of hydrogen bond network between colored forms by relatively weak hydrogen bond.