yellowing
A process or result of a gradual change from the
original appearance of a material, such as paper,
due to aging or environmental changes, or both.
While yellowing in paper is especially pronounced
in papers produced from mechanical wood pulps, it
occurs to a greater or lesser degree in virtually
all types of vegetable fibers. It is frequently
associated with brittleness in paper (although not
necessarily), and is sometimes referred to as
"color reversion." (17 , 72 )