Definition of colouring in in US English:

colouring in

(US coloring in)

noun

Australian, NZ, British
  • The action of filling in an outline drawing with colour using pens, crayons, etc., especially as done by a child.

Origin

1970s. From colouring + in, after to colour in. With colouring-in book compare earlier colouring book.

Pronunciation

colouring in

/ˌkʌlərɪŋ ˈɪn/