I don't think that GREEN was selected by many of the former slaves, the surname was given to many slaves from different counties over a period of time in North Carolina. In my ancestors case, their slaveowner, SAMUEL WILLIAMS (who never married), was also the father of my 4th & 5th ggrandfather, THOMAS GREEN, b.1760 (it's a long story, he's related on both sides);he freed THOMAS & his mother, CREASE, as well as THOMAS' wife and 5 children, and the 1798 Act to emancipate also freed the children that had been born since the 1791 will had originally been written. WILLIAMS' cousin, JAMES ALSTON, of Warren Co., was assigned the duty of seeing to it that his slaves were freed, and that they would receive their inheritance, as well as the fact that the rest of the family seemed well aware of the relationship because SAMUEL had 2 surviving brothers, and lots of nephews & nieces, all of whom lived nearby. If anything, I would have expected them to have adopted the WILLIAMS surname.
By the way, here is the entry for their emancipation:
(1798)
An Act to Emancipate Certain Persons Therein Mentioned (Pg.49)

I. Be it enacted by the General Assembly of the State of North Carolina, and it is hereby enacted by the authority of the same, That from and after the passing of this act, Rose the wife of Lemuel Overton, and her two sons John, Burdock, a Negro woman named Grace, and her son Harry, the property of Thomas Amis, deceased; a Negro girl named Bett, and a Negro boy named John, the property of Moses Parker; a mulatto girl named Nancy, and her child Eliza, the property of General Thomas Person, of Granville County; a Negro woman named CREASE; a Negro fellow named TOM, and his wife PRIS and her five children, to wit, ALLEN, CHARITY, BRENY, WILLIE and CREASE, the property of Samuel Williams, deceased, of the county of Warren, also the increase of the said Pris and her female children, since the date of the last Will of the said Williams; also one Negro man Daniel, formerly the property of formerly the property of James Allen; and a Negro man named Ginger, formerly the property of mark Allen, of Montgomery County, be and they are hereby emancipated and set free, and hereafter shall be called and known by the following names, to wit: ROSE OVERTON, JOHN OVERTON, BURDOCK OVERTON, GRACE WEBB, HARRY WEBB, BETTY BLACK, JOHN BLACK, NANCY HART, ELIZA HART, ELIZA HART, CREASE GREEN, TOM GREEN, PRIS GREEN, ALLEN GREEN, CHARITY GREEN, BRENY GREEN, WILLIE GREEN, and CREASE GREEN, and increase of the said Priss and her female children as aforesaid, by their respective names, with the addition of GREEN, DANIEL SHAD and GINGER PEPPER, and by the names aforesaid, they and each of them shall have, enjoy and possess all the rights, privileges and immunities which they would have been entitled to, had they been born free; any law to the contrary notwithstanding.
II. And be it further enacted, That a Negro man the property of Ozborne Jeffries, named SAM, and a Negro woman the property of Dixon Bogye, named CHELSEA, shall be and they are hereby emancipated and set free, the said Sam by the name of BUFFALO SAM, and the said Chelsea by the name of CHELSEA REED, and they shall be entitled to the same privileges, which persons of colour born free are entitled to.
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Deloris