I was just thinking of that one (Lord Thomas and Lady Ellender). I think "the brown girl" in that one is brown-haired rather than brown-skinned -- in contrast to the "fair" Ellender, but I could be wrong. Much the same sort of description applies to the "dark girl" who is the villain in many versions of "The Two Sisters." Her sister is also clearly a blonde, judging by her golden hair that is used to string the harp.