I think we're saying the same thing. One could charactarize the 'color' as gray. In short, when it's not black anymore, re-seal. Further down, I do mention that one should not wait until the emulsion between the stones is 'gone' or wearing off'. For SURE, if this is occuring, the stone is probably showing white!
Yes, once the 'black' is worn, re-seal. (Just don't wait too long.)