"A huge head bristling with red hair ..that little left eye obstructed with a red, bushy, bristling eyebrow."
Red hair is supposed to have the most malign influence, and has even passed into a proverb-"Let not the eye of a red-haired woman rest upon you."
(Ancient Legends, Mystic Charms, and Superstitions of Ireland, 1887.)
"Jem," I asked, "what's a mixed race child?"
"Half white, half colored. You've seen 'em, Scout. You know that red kinky-headed one that delivers for the drugstore. He's half white. They're real sad."
"Sad, how come?"
"They don't belong anywhere. Colored folk won't have 'em because they're half white; white folks won't have 'em cause they're colored, so they're just inbetweens, don't belong anywhere."
Colliss Harvey relates this "red kinky-headed"-ness to the phrase "beat you like a redheaded stepchild", speculating that originally the stepchild referred to denoted a child of mixed race. This is interesting as we've speculated on here recently about the possibility that red hair is the product of racial interaction; or more specifically: diverse gene pools. (For the latest iteration see here: Melting Pots - Part III - Eumelanin and Pheomelanin).
The story in To Kill A Mockingbird takes place in the first half of the 20th century, when attitudes to race were quite different. So it might be worth looking at other writings from the time to see if red hair pops up in other discussions about mixed race children.
Finally, another little nugget I liked was the following one. Apparently in medieval Germany one term for freckles was "Judasdreck" - which translates as "Judas-shit". We've noted the many quotes and depictions that portray Judas as red-haired on here before. Now we have a specific reference to the freckles as well. I think I'll stick with the term "sun kisses" though :)