Green’s Dictionary of Slang

1. a slim person [the shape of artists’ colour pastels].

1906

S. Ford Shorty McCabe 267: Miriam wasn’t what you’d call a pastel. She was built a good deal on the lines of an L-road pillar.

2. (US police) an unmarked police car [the usual colouring].

1989

J. Morton Lowspeak.