A colour variation across the body of a carpet, the result of a slight colour difference in the intensity of the dye at the time of weaving. While mostly the batches of wool for each carpet were dyed at the same time in the same solution but sometimes a weaver would run out of a certain colour wool. They would have had to dye more of the colour wool. Since dying is not done in a proper scientific way, but by sight, to the eye the colours would be the same, however, the intensities maybe different. Therefore, the reaction to washing, sun and environment would be different giving rise to a change of colour over time.
In tribal pieces this is considered desirable and giving the piece character. In some modern weaving they try and recreate this feature in rugs.