A method for measuring a target component in a sample by detecting a phenothiazine-derivative color, comprising: a step of producing an oxidizing substance or reducing substance from the target component in the sample; a color-developer-adding step of adding, to the sample, a color-developer that produces a phenothiazine-derivative color through oxidation-reduction; a step of producing a phenothiazine-derivative color through oxidation-reduction between the oxidizing substance or reducing substance and the color-developer; a step of detecting a phenothiazine-derivative color in the reaction system by measuring a light absorbance, and measuring a presence/absence or an amount of the phenothiazine-derivative color produced, wherein, in the detecting step, a phenothiazine derivative color is detected in the presence of at least one color substance selected from the group consisting of 5-hydroxy-1-(4-sulfophenyl)-4-(4-sulfophenylazo)pyrazole-3-carboxylic acid and a salt thereof, a compound represented by Formula below, and a flavonoid-based color: (in Formula: X is hydrogen, a halogen, sodium, or potassium, and Xs may be the same or different, and Y is hydrogen, a halogen, sodium, or potassium, and Ys may be the same or different); and a step of determining a presence/absence or an amount of the target component in the sample based on a result of the measurement, wherein the target component is glycated protein, and further wherein in the step, hydrogen peroxide is produced as an oxidizing substance by causing fructosyl amino acid oxidase to act on the glycated protein. a step of producing an oxidizing substance or reducing substance from the target component in the sample; a color-developer-adding step of adding, to the sample, a color-developer that produces a phenothiazine-derivative color through oxidation-reduction; a step of producing a phenothiazine-derivative color through oxidation-reduction between the oxidizing substance or reducing substance and the color-developer; a step of detecting a phenothiazine-derivative color in the reaction system by measuring a light absorbance, and measuring a presence/absence or an amount of the phenothiazine-derivative color produced, wherein, in the detecting step, a phenothiazine derivative color is detected in the presence of at least one color substance selected from the group consisting of 5-hydroxy-1-(4-sulfophenyl)-4-(4-sulfophenylazo)pyrazole-3-carboxylic acid and a salt thereof, a compound represented by Formula below, and a flavonoid-based color: (in Formula: X is hydrogen, a halogen, sodium, or potassium, and Xs may be the same or different, and Y is hydrogen, a halogen, sodium, or potassium, and Ys may be the same or different); and X is hydrogen, a halogen, sodium, or potassium, and Xs may be the same or different, and Y is hydrogen, a halogen, sodium, or potassium, and Ys may be the same or different); and a step of determining a presence/absence or an amount of the target component in the sample based on a result of the measurement, wherein the target component is glycated protein, and further wherein in the step, hydrogen peroxide is produced as an oxidizing substance by causing fructosyl amino acid oxidase to act on the glycated protein.