Black coats of the Asha'man

From A Wheel of Time Wiki

As a piece of clothing, this is the mark of anyone training to be Asha'man wears. Obviously, the coat is black, but the material varies. Some wear wool, some silk, and some (like Mazrim Taim) have embroidered the otherwise unrelieved black.

Because of the trick of not feeling the elements, the material is chosen based on personal preference and not on climate.