For a very minor cosmetic difference, I would probably just have the one listing for both. That is, if the packaging, the content of the cassette and the book are identical in every other way, as there would be no great benefit from two listings (eg anyone looking for that product probably won't use white or blue cassette as a search term, and I'd presume that perhaps if given a choice people may have a preference, but without it I doubt anyone would really care). I'd take a photo of one, and have a note in the listing letting people know that the cassette colour varies.
If they aren't identical in every other way, there would be other things you could use as differences - eg publishers, book illustrators, if the story is narrated by different people on the cassette etc
