Edit: This question is about chromophore production in lignin as found in low cost or short-term use paper such as newsprint. A different, excellent answer describes oxidation yellowing of the cellulose found in books.
This answer contains a quote from "Stuff Matters" by Mark Miodownik:
Lignin reacts with oxygen in the presence of light to create chromophores ( meaning, literally, 'color carriers'), which turn the paper yellow as they increase in concentration. This type of paper is used for cheap and disposable paper products, and is why newspapers yellow quickly in light.
I've read here that bleach and peroxide can oxidize chromophores by reacting with double bonds, reducing or removing the appearance of color. In this particular case of what sounds like a photochemical reaction of lignin with oxygen, how is a chromophore produced?
Or, is it really even true that a chromophore is responsible for the yellow color of yellowed paper? Or is it the oxidation and loss of a fluorophore that was originally making the paper look white? I'm not sure of that, because the phenomenon seems to be pretty old, possibly before the use of fluorophores in paper manufacture.