The spectral behaviors of two coloring matters (I and II) were investigated, which were formed in the color reaction of acetone with TNB. An equilibrium reaction system related with I and II was proposed as shown in Chart 4. We had found that the colorations of acetone with TNB in sodium hydroxide solutions were intensified by neutralizing the reaction mixtures with sodium dihydrogen phosphate. Now, the phenomenon was successfully explained by the scheme. On the other hand, another coloring matter (III) was isolated from the reaction mixture, which clarified the cause why the final absorption intensity at 480 mμ much increased when the conversion of I to II was carried out in the presence of excessive TNB. The structure of III, a tetracyclic compound, was determined as III' as shown in Chart 5. The correlations among the three coloring matters and their roles in the color reaction were discussed.