Why yellow can stand out without taking over
A highlighter has two jobs: make a passage easy to find, and leave the words beneath it easy to read. Yellow is bright to the eye but, in a thin transparent layer, usually lets the dark printed letters show through. That balance made it useful not just on the page, but when marked documents were copied.
What a black-and-white copier sees
A monochrome copier does not reproduce color as color. It measures how much light the page reflects, then renders lighter areas pale and darker areas dark. Yellow pigment reflects much of the light the copier uses, so a yellow stripe often registers as very light gray—or barely registers at all. The black letters, which reflect little light, remain dark.
This is a matter of contrast, not a special ability of yellow to vanish. A deep or heavily applied color can scan darker and muddy the text; a pale mark is less likely to do that. Copier settings, paper, and ink can change the result, so yellow is not guaranteed to disappear completely.
A practical choice that shaped the familiar marker
Fluorescent yellow can look especially vivid under ordinary lighting because it converts some ultraviolet light into visible light. A copier may not capture that extra glow, but the mark still looks bright to a person reading the original. Yellow therefore offered a useful split: conspicuous on the page, often unobtrusive on a monochrome copy. That made it a dependable default for notes that might be photocopied and passed along.
