This is a general point about question words: they don't have the same prime place in the sentence as they do in English. Rather, they are just part of the normal sentence structure you'd use otherwise. Here's an example with a different sentence structure but showing the same substitution:
I want the blue fruit: Makemake au i ka hua ’ai uliuli.
Which fruit do I want? Makemake au i ka hua 'ai hea?
Note that in either case, "hea" (which) just substitutes for the other adjective.