'two-colour' is a compound formed from the numeral 'two' and the noun 'colour'. 'Two' ultimately comes from Old English 'twa' (Proto-Germanic *twai), and 'colour' originates from Old French 'colour', from Latin 'color'.

'two' descended from Old English 'twa' (related to Proto-Germanic *twai). 'Colour' passed from Latin 'color' into Old French as 'colour' and entered Middle English as 'colour'; the compound 'two-colour' developed in Modern English as a descriptive compound (parallel to forms like 'two-tone').

Originally a plain noun phrase meaning 'two colours' (e.g. 'a flag of two colours'), it has been used adjectivally as a fixed compound modifier meaning 'two-toned'; the basic sense of 'consisting of two colours' has remained stable.