The present invention provides a phenothiazine-derivative color-measuring method that can detect a phenothiazine-derivative color even at a wavelength longer than the wavelength that exhibits maximum absorption. A phenothiazine-derivative color is detected, by adding a 5-hydroxy-1-(4-sulfophenyl)-4-(4-sulfophenylazo)pyrazole-3-carboxylic acid salt, 6-hydroxy-5-(4-sulfophenylazo)-2-naphthalenesulfonic acid salt, 3-hydroxy-4-(4-sulfonaphthylazo)-2,7-naphthalenedisulfonic acid salt, 7-hydroxy-8-(4-sulfonaphthylazo)-1,3-naphthalenedisulfonic acid salt, 3',6'-dihydroxy-2',4',5',7'-tetraiodospiro[isobenzofuran-1(3H),9'-(- 9H)xanthene]-3-one salt, 3',6'-dihydroxy-2',4',5',7'-tetrabromo-4,5,6,7,-tetrachlorospiro[isobenzo- furan-1(3H),9'-[9H]xanthene]-3-one salt, 4,5,6,7-tetrachloro-3',6'-dihydroxy-2',4',5',7'-tetraiodospiro[isobenzofu- ran-1(3H),9'-[9H]xanthene]-3-one salt or flavonoid-based color to the reaction system containing a phenothiazine-derivative color, and then measuring the light absorbance at a wavelength of 610 to 730 nm.