Green’s Dictionary of Slang

(Irish) milk as poured into tea.

1942		E. Cross Tailor and Ansty 33: ‘Will ’oo have “colouring”?’ she asks then. ‘Colouring’ is Ansty’s term for milk.
1967		B. MacMahon Honey Spike n.p.: ‘What do you want?’ the woman asked. ‘A drop o’ colourin’ ma’am.’ Breda held out the pint bottle [BS].