Four stages [of the alchemical opus] are distinguished … melanosis (blackening), leukosis (whitening), xanthosis (yellowing), and iosis (reddening) … the xanthosis, otherwise called the citrinitas, gradually fell into disuse
This passage establishes the foundational problem of the chapter: yellow (citrinitas/xanthosis) is the historically omitted fourth color of the alchemical opus, whose neglect collapses the fourfold symbolic system into a trinitarian one.
, Alchemical Psychology, 2010thesis