The first came out red [H132], all his body like a hairy cloak, so they called his name Esau.
admoni: red, ruddy
Original Word: אַדְמֹנִי
Part of Speech: Adjective
This Hebrew word is related to the name Adam and the color of the reddish earth.
It appears, therefore, that Esau's body was entirely covered with red down, which developed in time into hair as coarse as that of a kid (Genesis 27:16), and betokened a strong and vigorous, but sensual nature.
Now let's see David in 1 Samuel 16:12
English Standard Version
And he sent and brought him in. Now he was ruddy [H132] and had beautiful eyes and was handsome. And the LORD said, “Arise, anoint him, for this is he.”
Esau was red and David was red, the same Hebrew word for red.
Adam Clarke interprets this as red hair and complexion.
He was ruddy - I believe the word here means red-haired, he had golden locks. Hair of this kind is ever associated with a delicate skin and florid complexion.
The following two websites were pointed out by Ruminator in his comments:
The hair of the ancient Hebrews was generally black (comp. Cant. iv. 1, v. 11). In Eccl. xi. 10 black hair is designated as a sign of youth in contrast with the white hair of age. Josephus narrates ("Ant." xvi. 8, § 1) that Herod dyed his gray hair black in order to appear younger. Black hair was in any case considered beautiful, black being the general color, while light or blond hair was exceptional. David is designated as "admoni" = "ruddy" (I Sam. xvi. 12, xvii. 42), this expression being also applied to Esau's hair (Gen. xxv. 25). The Hebrews had thick hair (Ezek. viii. 3). Long, heavy hair was considered as a sign of vitality. In the case of Samson, traced back to religious reasons (he having been dedicated to God), the connection of long hair and bodily strength was based on the current views. Absalom's famous hair (II Sam. xiv. 25 et seq.) was considered not only as an ornament, but as a token of strength. A bald head, therefore, was an object of mockery (II Kings ii. 23; comp. Isa. iii. 17, 24).
https://jewishencyclopedia.com/articles/7061-hair#anchor8 show the prevalence of red hair among Jewish populations.
in Caucasia, where the natives are dark, the Jews show 96 per cent of dark hair. The proportion of red hair is also quite high, reaching 4 per cent in some observations. This has been considered characteristic of the Jews by some anthropologists. It appears to be not of recent origin, and was not unknown among the ancient Hebrews (Esau was "red, all over like a hairy garment"; Gen. xxv. 25).
The consensus among anthropologists seems to be that by far black hair was the dominant trait. Red-haired Israelites were possible exceptions.