'dual-tone' is a modern English compound formed from 'dual' + 'tone'. 'dual' ultimately comes from Latin 'dualis' (from 'duo') where 'duo' meant 'two'; 'tone' comes via Latin/Old French from Greek 'tonos' meaning 'stretch, pitch, tension'.

'dual' came into English from Latin 'dualis' (related to 'duo') through Middle French or Late Latin and has been used in English since the 17th century; 'tone' entered English from Old French/Latin influenced by Greek 'tonos' and developed into the sense of 'pitch' or 'shade', and the compound 'dual-tone' is a descriptive modern formation combining the two words.

Individually, 'dual' originally meant 'of two' and 'tone' referred to pitch or shade; combined as 'dual-tone' it came to mean 'having two tones' (applied to color, sound, or finish) without major semantic shift beyond composition.