The color sequence descends from Greco-Egyptian alchemy: Zosimos of Panopolis (c. 300 CE) already describes blackening and whitening in his visions and treatises. Medieval Latin authors codified nigredo-albedo-rubedo, with citrinitas variously inserted or dropped; the colors mark matter's death, purification, dawn, and completion. Zosimos's treatises were collected by Berthelot (Collection des anciens alchimistes grecs, 1888).
C. G. Jung made the sequence the backbone of his alchemy-as-psychology reading: nigredo as confrontation with the shadow, albedo as washing, rubedo as integration, in Psychology and Alchemy (1944) and Mysterium Coniunctionis (1955-56). Modern pop psychology's 'dark night before growth' owes its palette to this reading.
The colors encode alchemy's anti-optimism: the first sign of progress is putrefaction. Nigredo is not a prelude to the work but its opening success, the necessary death of the prima materia, which is why every tradition that borrowed the scheme kept the darkness first.
If transformation has a color sequence, what color is your current stage?