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[
isobenzofuran-1(3H),9′-[9H]
xanthene]-3-one salt, 4,5,6,7-tetrachloro-3′,6′-dihydroxy-2′,4′,5′,7′-tetraiodospiro[
isobenzofuran-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.