Wasei-eigo: odd eye
Heterochromia (from Greek ἕτερος (heteros, different) and χρῶμα (chroma, color)) refers to a difference in coloration, usually of the iris but also of hair or skin.
This tag usually refers to complete heterochromia iridum (both irises have a different coloration). For subjects with partial heterochromia (part of one iris has a different color from its remainder), use multicolored eyes. If the irises are not identical, then it is considered heterochromia.
Generally, include any eye-color tags for each instance. Don't use the multicolored eyes tag unless one or both individual eyes have more than color.