Morphology of Coloration in Sexy Librarians

Due to regulatory consolidation and union consistency work the sexy
librarian trope became more or less standardized in 1934. As of then,
to be a sexy librarian, you had to be a female natural brunette. This
standardized the common stereotype and met with little resistance.
There were a small number of angry blondes and redheads, picketing, who wanted these poorly-paid positions, but the stereotype held.

1950: The advent of Eastmancolor meant that the fine texture and highlights of blonde hair could be rendered sympathetically on screen, and the world changed. Regulations and safety rules were adjusted to permit natural blonde sexy librarians up to 0.8 Albedo on-screen and off. 

1964: The Ginger League achieved parity for redheads.

1972: Lobbying by the Bathhouse Boys created a revolution. Now male librarians could be sexy.

2023: Redheaded sexy librarians of either sex are not yet common. Albinos are disallowed because fire regulations prohibit Albedos of 0.95 and greater.