'dull-colored' is a compound of English 'dull' and 'color(ed)'; 'dull' comes from Middle English 'dul(l)' meaning 'blunt, dim', and 'color' comes from Latin 'color' via Old French 'colour'.

'dull' developed from Old/Middle English forms like 'dul(l)' meaning 'blunt' or 'faint', while 'color' passed into English from Old French 'colour' (from Latin 'color'); the hyphenated adjective 'dull-colored' arose in modern English by combining these elements to describe hue.

Originally 'dull' could mean 'blunt' or 'not sharp' and 'color' meant 'hue'; together the phrase came to mean 'having a hue that is not bright' (i.e., lacking brightness or vividness).