Quantitative HPLC and colorimetry are used to study color variations in dyeings with indigo, 6-bromoindigo, and 6,6?-dibromoindigo, the main components of the historic dye Tyrian purple. For the first time, visible light is identified conclusively as a cause of debromination of the leuco form of 6-bromoindigo. A dyeing run using 6-bromoindigo alone is found to yield a dyed fabric containing large amounts of indigo, when the vat is exposed to visible light. The extent of debromination is dependent upon the pH of the dye bath and also the source of the visible light. This information allowed development of a dyeing procedure which is demonstrated to give consistent colors through two passes. Quantitative HPLC analysis of extracts from the dyed fabrics indicates that the leuco form of 6-bromoindigo vs. the leuco forms of indigo and 6,6?-dibromoindigo has the strongest affinity for wool fabric. This is postulated to be due to attractive electrostatic interactions between the leuco form of 6-bromoindigo and wool.