Cotton and most other fibers do not start life WHITE. They are usually some shade of off-white or beige. The first step in making them WHITE is to wash (scour) it, the second step would be to bleach it and the last step, dye it with Optic whiteners.
All fabrics are washed, some are bleached and some are both bleached and optically whitened. In the industry, fabric to be dyed is not usually optically whitened because the optic whitener is like a white dye and occupies the same molecular space that the colored dye should occupy.
In our experience we can't tell the difference in the color of dyed fabric which had or had not been optically whitened. But that's just us.
We offer fabrics that have and have not been bleached and or optically whitened. They all dye just fine.