The Quadrille printing was an oily varnish applied on the front of the sheet before the stamp was printed .The squares measure 2.6mm.The Quadrille is found on the 15c stamps from 1891 - 4 Dec. 1900.
It looks like by aging and other factors the varnish discolored the surface ink.
In certain printings for France & colonies the paper was very lightly tinted.
Just a theory; perhaps the varnish was not dry and the sheet was printed upon creating a
chemical change in the suface ink over time.
There were over 14 billion 15c stamps printed.Of course many types are found in the course of it's 22 year lifespan.
Nice example from Djibouti:http://goscf.com/t/13125