The four Latin stage names of the Great Work translate an older Greek sequence, and the older names are worth knowing because they are plainer.
The Greek names
The Graeco-Egyptian alchemical corpus describes the work as a series of colour changes: melanosis the blackening, leukosis the whitening, xanthosis the yellowing and iosis the reddening. Albedo renders the second of these. The Latin terms in general use today are translations of that sequence rather than an independent scheme.
What the whitening was
In the Greek material this is a stage in the treatment of a metal, and whitening is a describable, repeatable change in the appearance of a worked surface. The practical operations behind it, involving mercury, arsenic compounds and heat, are recognisable as metalworking technique.
That the same sequence was afterwards read as a moral progression, and later again as a psychological one, does not remove the first reading. The atlas records the colours as the sources give them and the later interpretations as later.
Where the sequence points
The whitening is the midpoint of the four and, in most accounts, the first result that could be shown to anyone. Several traditions treat it as a lesser completion in its own right, a work that could be stopped at rather than a step that must be passed through.