Where does “Ἐρυθραῖος” come from?

Ἐρυθραῖος (Ancient Greek) comes from Ancient Greek Ἐρυθραί, from Ancient Greek ἐρυθρός, from Proto-Hellenic *erutʰrós.

Ancestry of “Ἐρυθραῖος”, step by step

Step	Language	Word	Meaning
1	Ancient Greek	Ἐρυθραί	—
2	Ancient Greek	ἐρυθρός	reddish, red
3	Proto-Hellenic	*erutʰrós	—

Ἐρυθραῖος — Etymology, Origin & Word History | Etymology Explorer