Green’s Dictionary of Slang

(a) a mixed-race couple, thus friends or colleagues of different races.

1954		W.G. Smith South Street 266: For Kristin’s sake, Claude had made the supreme effort to control his temper. [...] Passing them, on the street, people frequently made comments: ‘Well, look at that! Salt and pepper!’.
1970		D. Ponicsan Last Detail 159: ‘Hell, Boston is full of sailors.’ ‘Yeah, but how many of them are salt and pepper, toting two .45’s in an AWOL bag?’.
2001		Indep. 11 July 3: A former manager of a McDonald’s restaurant [...] told an employment tribunal in Reading, Berkshire, that white and black people were represented by the words ‘salt’ and ‘pepper’.
2001		J. Stahl Plainclothes Naked (2002) 124: Says it was two guys, a salt-and-pepper.
2021		S.A. Crosby Razorblade Tears 155: ‘Here’s the deal, Salt and Pepper’.

(b) a police team, usu. operating from a squad car, that consists of one black and one white police officer.

1975		L. Dills CB Slanguage.
1980		E. Folb Runnin’ Down Some Lines 252: salt and pepper 1. Police.
1989		J. Morton Lowspeak.

(c) a black and white squad car.

1980

E. Folb Runnin’ Down Some Lines 66: The vehicles and flashing lights that identify their continuous presence in the lives of blacks (black and white, salt and pepper).