In my experience (most recently today, when some of a Madagascan collection I bought has been discoloured by the stock-book), toning is there to stay. It will reduce value, but not usually destroy it - I almost expect it on older French engraved stamps.
Conservation means preserving stamps, not restoring them. You could restore the paper to whiteness with various chemicals but this would negatively impact the stamps and their value. It would also open the door for documenting what you have done and disclosing it if the collection ever passes from your hands. Most people want to know if the stamp and covers they are getting have been chemically altered or 'restored'.
Sounds like you have done what most people do, which is remove them from the acidic conditions (poor quality stock book) and the toning (browning) of the stamp edges. Also a cool, dry and stable storage environment is paramount; temperatures should be held at a constant 70°F with a relative humidity held between 30% and 50%. Don
Many hobbies value the aging of an item (i.e. numismatics, antiques, etc.) but I am unsure why this hobby does not seem to have the same perspective. Even the old car hobby is now clear coating old, faded paint original paint jobs as opposed to restoring a car. Don