Abrash is an Arabic word commonly used in the oriental rug industry.
For variety of reasons they never prepared the necessary amount of the yarn for an entire rug or carpet at one time. The yarn was made in small batches to accommodate the dying vessels and in most cases the colors matched perfectly. However many times because of the slight difference in wool and dyestuff quality the aging of these yarns differed with the time. Therefore in most old items you will find streaks of colors, since the rugs are made row by row. This phenomenon is so often that in many reproduction carpets they create abrash by purpose.