Citrinitas, the yellowing, is the third of the colour-stages through which the alchemical Great Work (magnum opus) was said to pass. It names the ripening of the white matter toward a golden yellow.
The colour-stages of the Work
Symbolically Citrinitas marked the dawning of a solar, mature wisdom; in later schemes this stage was sometimes folded into the others and the sequence reduced to three. The classic sequence, nigredo, albedo, citrinitas, rubedo, describes the transformation of base matter into the perfected end of the Work through a succession of colours seen in the vessel.
Citrinitas is the stage most often dropped from the sequence in the later Latin tradition: by around the fifteenth and sixteenth centuries many alchemical texts work with a threefold black, white, red sequence and treat the yellowing as a brief, transitional shade within the passage from white to red rather than as a distinct fourth stage in its own right, which is one reason it receives comparatively little independent discussion compared with the other three. Where it is kept as a separate stage, it is generally read, including by the depth psychologist C. G. Jung in his 1944 study Psychologie und Alchemie, as a dawning solar consciousness distinct from albedo's cooler, lunar clarity, a bridge stage rather than an end in itself. The stage is described here within that documented tradition of European alchemy and its later interpretation.
Sources The Secrets of AlchemyLawrence M. Principe with Psychology and AlchemyC.G. Jung