The coloration phenomenon of L-tryptophan (Trp) treated with trifluoroacetic acid (TFA) was observed under varying pH, i.e., from yellow (above pH 5.5) to red (below pH 4.0). In order to elucidate the coloration mechanism, the coloured compounds were fractionated and investigated spectroscopically. It was found that Trp forms a yellow compound consisting of a tricyclic structure, and that the change in colour with the variation of pH is due to the dissociation of the nitrogen atom in an indole ring. Coloured copolypeptide hydrogel containing Trp residues was also prepared by a method similar to that for Trp. The colour change was also observed for the hydrogel. The FH dependence of such membrane properties as degree of swelling, solute permeability, and the mechanical property was investigated. These properties changed drastically in the pH region of the colour change. This can be explained by the dissociation of Trp residues.