A sallow complexion means a yellowish complexion. It also applies to dark complexions like those of a mulatto, a gypsy, and other so designated olive and Mediterranean complexions. I suspect it also applies to the brownish-greenish complexioned Europeans who are usually depicted in Russian and Orthodox paintings of kings, biblical personalities and saints.
The fact that Europeans of the past were not as light-skinned as many of them are today is something that Europe's political and academic establishment choose to gloss over if not deny outright, but then again you see a film in which a mixed race person plays the role of a royal, such as Angel Coulby playing the role Queen Guenevere in King Arthur, or Sophie Okonedo playing the role of Margaret of Anjou in the Hollow Crown. Some of those thespians know better.
Because yellow skin is one of the features of jaundice the word became associated with illness and unhealthy looking skin even if the person was not ill or had jaundice.
Here are some quotes from the online Merriam Webster dictionary on the etymology of the word sallow - Merriam Webster Online Dictionary - Origin and Etymology of sallow
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There is no hint of sickliness in the etymology of sallow. The word
appears in Old English as salu or salo, and could mean "dusky" or
"dark" or "grayish greenish yellow." Salu (or salo) is akin to Old
English sōl ("dark, dirty"), Old High German salo ("murky, dirty
gray"), Old Norse sölr ("dirty"), and even Sanskrit sāra or
sāla, which carries the basic meaning of "dirty gray." Sallow, however, has for much of its history been used specifically to
describe the skin or complexion of one who is unwell.
Origin and Etymology of sallow
Middle English salowe, from Old English salu; akin to Old High
German salo murky, Russian solovyĭ yellowish gray
My own suspicion is that sallow complexions in the yellowish-greenish sense were the mulatto offspring of the people of the paintings described above with "white" people, which is probably why many European royals were described as sallow.
Henry VII, Queen Elizabeth I and her mother Anne Boleyn were described as having sallow complexions.
The effigies of Eleanor of Aquitaine and Henry II at Fontevraud Abbey, France.
Henry's complexion is what would be described as
sallow. Eleanor's complexion would be
ruddy. Sophie Okonedo playing Margaret of Anjou is not that far of the mark.
The word sallow probably referred to people who in current times would be classified as
black and the term carried over the their light-skinned descendants just as today the word black is used to label people showing visible signs of black African ancestry even when they are quite light-skinned, such as Meghan Markle.
Paintings of sallow-complexioned people are usually assumed to be those of pink-skinned white people in which have darkened or deteriorated over time and need to be
A good example of people in the public eye who can be described as sallow or display a sallow cast to the complexions every now and then are Viktor Orban, Maria Vladimirovna and Maria Sharapova.