Abrash is a change or variation in the color of a rug due to differences in the wool or dye bath. I some cases abrash is only a small shading difference but sometimes abrash can be quite distinct.

Early Kansu Carpet
An exclusive 18th century Kansu carpet with abrash. Size: 9 ft 4 in x 14 ft 2 in (2.84 m x 4.32 m). Nazmiyal Collection.