The active colouring constitutent of saffron is crocin, which is a di-gentabiose ester of crocetin, although the exact structure is not given. The dye is obtained from the stigmata of Crocus sativus, which are collected by hand, so the dye is expensive. Although used in the past for many purposes, today saffron is used primarily as a food colour and spice. In histotechnology, it is largely confined to the hematoxylin, phloxine, saffron (HPS) staining method, in which it colours connective tissue yellow in contrast to the pink cytoplasm of phloxine. Due to the expense of the saffron the method is not common, and modifications employing tartrazine are sometimes substituted.