Where does “Λευκορωσία” come from?

Λευκορωσία (Greek) comes from Greek λευκο-, from Ancient Greek λευκός, from Proto-Hellenic *leukós.

Ancestry of “Λευκορωσία”, step by step

Step	Language	Word	Meaning
1	Greek	λευκο-	API + πίνακας → λευκοπίνακας; API + ντυμένος ...
2	Ancient Greek	λευκός	bright, shining, gleaming; light in color; white;...
3	Proto-Hellenic	*leukós	—

Λευκορωσία — Etymology, Origin & Word History | Etymology Explorer