It definitely doesn't need to be inverted!
I think the second card was black in the example just to make the difference more obvious, but the contraption will use the appearance of the second card as it is. Whether it's normal 1-bit, inverted from your original or in color... anything should work just fine.
So, yes, using the dragable contraption as a magnifying glass to detect the small differences is absolutely a way you could use it!