Why yellow marks disappeared on early photocopies
A copier does not understand that a stripe is meant to be a highlight. It records differences in reflected light. In a typical black-and-white copier, the page is illuminated and the machine turns the brightness of the returning light into a light-or-dark image that can be printed with black toner.
Yellow highlighter ink usually reflects a great deal of light—especially compared with the dark letters beneath it. Against white paper, the yellow stripe can therefore look nearly as bright as the unmarked page to the copier. The machine preserves the darker text but may record little or no difference where the highlight lies.
The contrast problem behind the vanished stripe
The key is not the color’s name but its brightness as the copier detects it. A mark that absorbs more light, such as a dark blue or black pen, creates a stronger contrast with the paper and is more likely to show up. Yellow ink absorbs relatively little of the light that makes the page appear bright, so the copier can effectively treat it as blank.
This made highlighting useful on the original but unreliable in a black-and-white copy: readers could see the emphasis on the marked page, while someone relying on the reproduction might not know it had ever been there. Color copiers can preserve color information, but ordinary monochrome copying still reduces a colored mark to a question of contrast.
