I'm guessing because he's not sure if they'll work with a black car having not seen before. Aside from being able to test fit, it's not always easy to tell if it will look good. I thought it was a fair question.
In my opinion (take it or leave it), white wheels work best on cars with solid colour (i.e. not metallic), black, white, or LM blue as on special edition R33 GTR. Google image search also helps.