The Collaborative International Dictionary of English v.0.48:

Yellow \Yel"low\, v. t. [imp. & p. p. Yellowed; p. pr. & vb.
   n. Yellowing.]
   To make yellow; to cause to have a yellow tinge or color; to
   dye yellow.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Yellowing \Yel"low*ing\, n.
   The act or process of making yellow.
   [1913 Webster]

         Softened . . . by the yellowing which time has given.
                                                  --G. Eliot.
   [1913 Webster]