My best example is a line drawing of a skull or skeleton, and the "why the heck is my skull black" anecdotes.
My basic explanation is pretty much "the more realistic the black art, the less likely that it will look good in white without serious tweaking. I remind them of how creepy a film negative of a face can look.
In fact, just last Friday I was just asked to print a one color design in white that included an American flag. I couldn't help the fact that the stripes would be reversed, but I did invert the stars and their field. (and charged for the art time)
Long ago, was at a place that printed some black jackets for a group of men who, let's say' were "diversity challenged". They provided the art which was line drawings of them, but with the instructions to print it in white. Except that this could not have happened to a more deserving bunch of fellows, it now does remind me of the importance of mockup proofs!.