Definition of colour line in US English:

colour line

(US color line)

noun

  • Originally and chiefly US. An enforced separation between white people and black people (or occasionally those of other racial groups) in a country, community, or establishment.

Origin

Mid 19th century (in an earlier sense). From colour + line.

Pronunciation

colour line

/ˈkʌlə lʌɪn/