As characterization and narrative, the motif or archetype of "The Fair Lady and the Dark Lady" appears deeply embedded in western literature, art, and culture. The concept is potentially racist, as many of its sources admit, so be ready to recognize it but be careful in using.
According to this motif, physically the Fair Lady is white- or rosy-skinned with blonde hair, blue eyes, and adolescent or virginal figure. The dark lady, in contrast, usually appears darker-skinned, brunette, brown-eyed, sometimes with fuller figure.
Spiritually, the Fair Lady appears angelic, while the Dark Lady may appear as a temptress.
The male counter-pairing may be characterized as the Golden Boy and the Byronic Hero. Think Brad Pitt vs. Dracula.
In Sir Walter Scott's medieval romance-novel Ivanhoe, Rowena is descended from Anglo-Saxons, while Rebecca is a Jewess.
In James Fenimore Cooper's The Last of the Mohicans, Alice is descended from the Scots while Cora is part-Scots, part African American.
In Poe's gothic story Ligeia, the title character is the Dark Lady, while the narrator's victim 2nd bride Rowena is the Fair Lady