Several progeny (children, grandchildren, and great-grandchildren) of the Hemings/Jefferson relationship did in fact have Thomas Jefferson's trademark red hair; after leaving Monticello they lived openly as white, married white spouses, and personally identified as white, not black or colored. The progeny of the Hemings/Jefferson relationship were, in fact, 7/8 Caucasian European--and several were considered exceptionally handsome or beautiful.
Based on actual people and true events, although fictionalized with additional characters, events, and embellishments.
Early in the film, when Thomas Jefferson (Sam Neill) tells Sally Hemings (Carmen Ejogo) that she looks exactly like his deceased wife, this is a reference to Martha Wayles Skelton Jefferson being the half-sister of Sally Hemings, sharing father John Wayles. Martha was Wayles' acknowledged daughter by his first wife, Martha Eppes, and Sally was Wayles' daughter by his slave concubine, Betty Hemings (portrayed by Diahann Carroll), with whom he had six slave children.
The character of "Young Tom Hemings" (Jesse Tyler Ferguson), portrayed as the firstborn child of Sally Hemings and Thomas Jefferson, was a fictional character. None of the six children, born in the United States (there is some evidence of a child born in France, who died soon after they returned to Virginia), as a result of the relationship between Sally Hemings and Thomas Jefferson were conclusively named Tom or Thomas; the only child even possibly named Tom or Thomas was the sixth-born, commonly known as Eston Hemings, not the firstborn. The first child to survive into adulthood was Beverley Hemings, a boy (possibly named William Beverley Hemings 1798-c. 1873). Beverley was also the first child of the Hemings/Jefferson relationship to leave the plantation, ostensibly "running away" at age 24, to be joined later that year by his sister Harriet, who was openly released by Jefferson, given $50, and put on a northbound stagecoach by plantation overseer Edmund Bacon. The only known contemporary account/claim of a Hemings/Jefferson child to be born before 1795 and to survive into adulthood, was by James Callender (portrayed by Rene Auberjonois), known to bear a grudge against President Jefferson because of not being appointed as U.S. Postmaster of Richmond, as alluded to in this movie, and the name of that child was supposedly Thomas C. Woodson.
A third child, female, born 1799, died in infancy. The fourth child was again named Harriet (historians refer to her as Harriet [II], 1801-c.1863). The fifth and sixth children were both boys, Madison (possibly named James Madison Hemings, 1805-1877), and Eston (possibly named Thomas Eston Hemings, but not referred to as such by historians, 1808-1856).