I like the restoclean idea. I hate messing with blocks and small pieces, but if you are going to have to add some fabric anyway, maybe take out a piece of the existing white and see what happens when you treat it.
Tea dying is easy and really quite stable once you've dried it (and you can use coffee too, slightly different shade) and there are other easy things to do -- but getting the exact results you want can be super hard.
When I finish UFOs, I usually give a subtle sign that my hand has been there, so I will put in a different color of thread or something that may only be visible on the back. Sure, it's nice when I do have the perfect/exact fabric but when I don't, I go ahead on the principle that "it may be visible but that's ok". Sometimes visible is subtle and sometimes we just make up something entirely different to fill the gaps. It's part of the story of the project.