A purple color is formed during the fibrillation of lysozyme, a well-studied protein lacking a prosthetic group. The application of Raman spectroscopy, electron paramagnetic resonance and UV-vis absorption spectroscopy indicates the formation of a sulfur. pi-bonded radical cation due to the methionine-phenylalanine interaction, which is consistent with a small molecule model reported in the literature. A purple chromophore with characteristic 550 nm absorption is formed due to a specific orientation of the sulfur-centered radical cation and a phenyl ring stabilized by the fibril framework. A specific fibril conformation and the resulting formation of the chromophore are controlled reversibly by varying the pH. This is the first known example of a side chain self assembled chromophore formed due to protein aggregation.