Hair dye is generally classified into four types, namely, oxidative hair dye, ionic hair dye, temporary hair dye, and others. Among these hair dyes, an oxidative hair dye, which has been most widely used at present, is also referred to as a permanent hair dye, and it is mainly constituted with paraphenylenediamine or para-aminophenol that becomes an active reaction intermediate as a result of oxidation by hydrogen peroxide. The active intermediate then reacts with a dye coupler molecule in hair, and it changes to a shampoo-resistant hair dye. However, such an oxidative hair dye damages hair, may cause contact dermatitis or latent influence on total body for a long period of time, and may be suspected as mutagenicity or carcinogen. An ionic hair dye is also referred to as a semi-permanent hair dye, and it does not damage hair. However, such an ionic hair dye is problematic in term of skin coloration upon dyeing, and in that the dye is washed off by performing shampooing operations four to ten times. A temporary hair dye does not damage hair, and skin coloration is overcome by washing. However, such a temporary hair dye is washed off by performing a single shampooing operation.
As another hair dye, there has been proposed a non-oxidative hair dye containing polyvalent phenol and an iron salt (Patent Documents 1 to 3). However, previous non-oxidative hair dye products have not been satisfactory in terms of hair dyeing property and color tone, and have been problematic in terms of loss of color due to shampooing. In addition, the stability of an iron salt is low in an aqueous solution, and thus the iron salt is easily precipitated in a solution. As a result, it is difficult to produce a liquid-type non-oxidative hair dye containing an iron salt.
Moreover, it has been known that an organic acid is mixed to control a chelating reaction. As a result of the mixing of an organic acid, coloration of the skin is reduced. However, it has been problematic in that significantly decreased reactivity may cause degradation of the hair dyeing property (Patent Documents 4 and 5).