Here is what I read on the subject.
Natural cotton, after weaving and washing, is an off-white color like a light shade of ecru or cream. Normally, if the fabric is to be dyed, it goes straight to the dyer at that point. If it is to be sold as bleached fabric or made into "white" clothing, it is first bleached, washed, then often treated with optic whiteners and washed again. Optic whiteners are kind of like a white dye. While technically there is no such thing as a "white" dye, the optic whiteners occupy on a molecular level, the same spaces as dyes do. Therefore, natural, unbleached fabrics are said to take dyes better than "white" or optically whitened fabric.
So your ready to dye fabric is simply fabric without the optic whiteners added to make it look like the bright white of white cloth. Also it normally won't have any sizing in it. Otherwise it is exactly the same cloth as any other. There is nothing in it that attracts color.