Thus, when the surface abrades, the dyed part of the yarn is gone and its natural color peeks through. In fact, they don't. If you've struggled with grass stains on kids' clothes, you might be surprised to learn that natural green dyes are fairly rare. Some very early Turkoman pieces have a natural dark green in their palette, but most of the greens that are seen on rugs made prior to the availability of synthetic dyes are produced by consecutively dying the yarn with indigo and a natural yellow. These are usually easy to identify.
Some natural yellows are very sensitive to light.
In pieces using these dyes the color on the surface exposed to light will be blue while the back of the piece will show that it was once green. Blacks and dark browns were sometimes derived from sheep with wool in those colors prior to the availability of synthetic dyes, but most often they were derived from dyes with corrosive effects on wool. The dark brown or black areas will be more worn than the others, giving a relief effect. In many old rugs the blacks or dark browns are worn to the point that the pile is completely gone from those areas. One way to distinguish these from natural corroded blacks is that the wear in the areas dyed with corrosive blacks is variable from one part of the rug to another.