"What color is his hair?"
Translation:Jakiego koloru są jego włosy?
Is this genitive or accusative? And also why not instrumental since the verb być is involved? Thanks :)
I replied: Jakiego koloru jego włosy? It was not accepted. Is 'są' necessary in this sentance?
Yes, it is necessary. I think the only construction where you can (sometimes, at least) omit a form of "być" is "This is Y" (To jest duży pies vs To duży pies).
Unlike Russian, Polish just needs it.