Citrinitas is the stage of the Great Work that most later accounts do not have, and its disappearance is the most interesting fact about it.
The Greek sequence had four
The Graeco-Egyptian alchemical writings describe the work as four colour changes: melanosis, leukosis, xanthosis the yellowing, and iosis. Citrinitas is the Latin for the third of these, and it is securely part of the older scheme.
Three, not four
By the later Latin tradition the yellowing has in many authors been absorbed into the stages on either side of it, leaving a sequence of three: black, white, red. That three-stage version is what a great deal of later writing uses, including much of the modern psychological literature that borrows the vocabulary.
Neither version is wrong. They are different periods of the same tradition, and a reader who meets three stages in one book and four in another is not looking at an error.
Why it went
No single reason is documented. Proposals include the practical difficulty of distinguishing a yellow stage from the reddening that follows it, and the general tendency of a threefold scheme to be easier to teach than a fourfold one. The atlas records the four-stage sequence because it is the older, and records that the three-stage version is at least as widespread.