I think the RIT products are a good idea. Another rust removal chemical is CRL.
There a number of phenomena that discolor fabrics- mold, gas fading and discoloration due to air pollution or fumes from furnaces or nearby industries. Some of theses cause a chemical change in the fabric or the sizing that can not be easily removed
Just a word of caution- some of theses chemicals are acid based and should not be mixed with chlorine bleach. Even if your fabric is washed out and rinsed there could be enough residual chemical to liberate irritating or poisonous gas into the air. Some de-rusting agents contain muriatic acid which is a bad when combined chlorine
If you are not conversant with all of this- it simply may be time for a new background and as suggested, paint this one another color.
Be careful- good luck! Ed
