'heterochromia' originates from Greek, specifically the word 'heterochrōmia', where 'hetero-' meant 'other' or 'different' and 'chroma' meant 'color'.

'heterochromia' passed into New/Modern Latin as 'heterochromia' from Greek 'heterochrōmia' and was borrowed into English with the same form and meaning.

Initially it meant 'difference in color' in a general sense, and over time it became a technical term most often used for differences in iris coloration (and occasionally for other tissues).