How do I best turn my yellow cotton sofa cover to a neutral stone
colour by bleaching?

Unless this is a throwaway sofa that you don't care about, I wouldn't
attempt to bleach it. You're likely to ruin it. It is very difficult
to apply bleach or dye evenly to an upholstered surface. I wrecked a
chair once trying to dye it. It ended up looking blotchy and smelling
funny, and the padding never recovered from having been dampened by
the dye.

Maybe you could leave it out in the sun? This might weaken the fabric,
but at least it wouldn't have the blotchiness and dampness problem.

Even if the cover is removable/washable, I expect that bleaching would
be less than satisfactory, especially if the cover has corded seams. 
If it is removable, you could ask a professional, but this site warns
against washing and bleaching for other reasons:  shrinkage and
deterioration of backing fabric.

http://centuryfurniture.com/ProductCatalog/ProductCare/Upholstery.pdf