The unique reddish blood stains on the archaeological cloth known as the Shroud of Turin caught the attention of several scholars, who proposed different hypotheses to explain the unusual blood color. To date, just a few hypotheses have been tested experimentally, and the results are debatable. In this paper, we test the strength of two hypotheses (namely, the presence of carboxyhemoglobin and the long-term influence of ultraviolet light on high-bilirubin blood) by the spectral reflectance of the blood stained regions on the Shroud and by color analyses of ultraviolet irradiated high-bilirubin blood stains on linen. The relevance of these simple methods to the study of stained textiles is discussed. © 2018 Optical Society of America.