'single-coloured' is a Modern English compound formed from 'single' + 'coloured'; 'single' ultimately comes from Latin 'singulus' meaning 'one' (via Old French), and 'colour/colour' comes from Old French 'colour' from Latin 'color' meaning 'hue'.

'coloured' developed from Middle English 'colour' (from Old French 'colour', from Latin 'color'), while 'single' developed from Middle English 'singel' (from Old French 'sengle', from Latin 'singulus'); these elements combined in Modern English to form 'single-coloured'.

Originally the components meant 'one' (single) and 'hue/colour' (colour); over time the compound came to be used simply to describe something 'having one colour' (i.e., monochromatic).