Why is it not "Hvad" instead of "Hvilken"?
"Hvilken" is the proper word grammatically but in daily speak most people would probably say "Hvad". It would, however, annoy linguists.
Dine øjne har farve - de er ikke farve. Men man kan sige : af hvilken farve er dine øjne. Mine øjne er blå - de har farven blå.