It sounds, maybe, like a problem in psychophysics
Colin is doing some interesting experiments. You will want to read How infuriating. LIRA (The Linen Industry Research Association of Belfast) is no more:
. . . I’ve been eye-balling what happens when one applies sticky tape to scorched linen. The first pull takes off heavily scorched fibres (not whole threads, note, but individual, so-called ultimate fibres). With fresh tape on the same area, one gets progressively lighter harvests of detatched fibres. So far so good. One is basically seeing what Raymond N.Rogers did with his sticky tape sampling of the Turin Shroud.
It’s what happens next that is interesting. If one takes the sticky tape samples, one can lever up free or broken ends of fibres, and then pull them out with tweezers (tricky but feasible). When one looks at the extracted fibres, one’s first thought is that they are colourless, matching Rogers’ description, i.e. his claim that the image colour stays behind through being highly superficial and able to be easily stripped off. But here’s the caveat. If one sticks the collected "clean" fibres back on paper with the same sticky tape. one then finds they are in fact still yellow or brown, and indeed is able to compare them with those that were not removed, i.e. still in situ, to see there is really no colour difference when compared side-by-side under the same conditions, i.e. white background, viewed through a thickness of sticky tape. In other words, one has to beware of artifacts when looking at individual fibres, even with the naked eye (with still more artifacts possible when using a microscope). . . .
Is it a form of the checker shadow illusion? The square A is exactly the same shade of grey as square B.
The image above by Adelson, Edward H. (2005). Called the "Checkershadow Illusion", it is found at MIT.edu. It is Licensed by Wikimedia wherein it is stated: The copyright holder of this work allows anyone to use it for any purpose including unrestricted redistribution, commercial use, and modification.