"What color is his hair?"

Translation: Jakiego koloru są jego włosy?

[IE99]

Is this genitive or accusative? And also why not instrumental since the verb być is involved? Thanks :)

[mihxal]

Genitive. I've explained it earlier: https://www.duolingo.com/comment/12338495.

[MichaelFivez]

Is "jaki kolor mają jego włosy" correct as well?

[Jellei]

Yes. Added now.

[Ksenija214148]

I replied: Jakiego koloru jego włosy? It was not accepted. Is 'są' necessary in this sentance?

[Jellei]

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.

[Arnoldpitt]

Where can I find an explanation of the different ways of making something plural, particularly difference between -y/-I and -ow (o usually with a tai)

[Arnoldpitt]

Sorry accent not tail!

[br0d4]

If you mean writing, I think that http://sgjp.pl/ might be a great help to you. It is by far the biggest dictionary of Polish online (with 455.560 entries and counting). It does not define the meaning of words (just shortly, sometimes) - but it lists ALL their grammatical forms; for words that may have different meaning (e.g. the same word being a common noun/a male family name/a female family name ) - it has as many entries as necessary in given case. For instructions, see here: http://sgjp.pl/instruction/