'multicolour' is a compound of the prefix 'multi-' and the word 'colour'. 'multi-' originates from Latin, specifically 'multus', meaning 'many', and 'colour' comes via Old French 'colour' from Latin 'color', meaning 'colour.'

'multi-' (from Latin 'multus') combined with English 'colour' (from Old French 'colour' < Latin 'color') to form the compound adjective 'multicolour' in modern English.

Initially it simply combined the elements to mean 'having many colours', and over time it has retained that basic meaning with little semantic change.