The grey skin or tooth discoloration is more common with minocycline but is also likely but to a lesser degree with doxycycline, tetracycline and lymecycline(don't laugh, there is a such a thing). If you take a capsule of minocycline and dump it out, you will notice that it is usually white in color unless it is old stock. Through time, as the chemical is exposed to sunlight and air, it turns black.
I'm surprised many llmd's fail to warn patients of this somewhat bizarre phenomena. And they even know you are taking the drug for extended periods beyond 2 weeks. Maybe they just don't know or care to. It is not life threatening, so that may be why.
They only way you can prevent the possible grey discoloration is with Vitamin C 500-1000mg per day, alpha lipoic acid and possibly some other antioxidants. They have the ability to stop this oxidation process. As they are considered antioxidants.
I did years of doxy, tetra and mino in the past but i took the above mentioned antioxidants along with them to prevent such a thing.
For the current grey tooth problem. I recommend a whitening toothwash such as listerene or generic brands. They contain a safe amount of hydrogen peroxide and certain surfactants to break up stains and plaque. You usually rinse your mouth with them for 1-2 mintues right before brushing. If you want to take this even further, get an electric toothbrush such as an oral-b or sonic. Their cleaning ability is far superior to conventional toothbrushes. Crest even makes a "once per week" polish that you use with the electric tooth brushes. It is very aggressive compared to typical tooth paste and more similar to what dental hygenists use. Hence, the reason you only want to use it once per week. Use it too much, and it could remove too much tooth material.
As for the skin, there may be some methods to remove the dark pigmentation but usually time will take care of it. Plus the addition of antioxidants to prevent future discoloration.