In my experience, once the gray color is pretty prevalent, the yeast is not worth reviving...but that is just my own standard based on fear of wasting time and effort - YMMV and in theory, if there is life in there, you should be able to regenerate the yeast cell count. Problem is, how would you know if it's healthy yeast (predominately)? Tom is right - smell is a great indicator in the absence of sophisticated measurement tools. Gray usually equals funky to me (and not in a good funky way).
Good luck!