There is quite a lot of information about muddy colors in the painting world as muddy colors are used by painters. Basically it is any color that has had gray or black added. Muddy colors can be any color and can range in value from light to dark. So when you are looking at fabric first pick a very clean clear color and then compare that to other fabric of the same color family to get the contrast of a more "muddied" color and see if that helps.