Some carpets have different shades/nuances, is there a fault or defect in such carpets?
No, there is no fault or defect in these carpets. What you notice is known as an Abrash (can be seen in certain carpets only). An Abrash is a Turkish term that means partly colored. This difference in nuances is generally found in nomadic (tribal) carpets, and it occurs when a weaver uses yarn from a dye that was not done in the exact same shade as the previously used.
