Sometimes it can take me a while to answer questions, but I'm always happy to do it--even though I can take a while. And you ask an excellent question! It's something that I will need to be thinking about soon, too: what to do with my hair as I get more gray hairs, and what is least damaging.

I can tell you that staying your color, or going a bit darker is much less damaging than going lighter. And if your hair is in very healthy, natural shape, coloring it a darker color shouldn't be too bad. I'm not sure about "covering grey" formulas, though, whether they might have a bit more ammonia or peroxide to make them "stick" to the greys. But any chemical process will damage our hair a little. The key is to try to minimize the damage to the barest minimum, balancing need with damage.

Another option is to go with a demi or semi permanent color. These have minimal damaging ingredients, but then you will have to do it over and over again, so that might not be practical.

What I will probably do when the time comes (so this is my recommendation for you as well) is find a color that's very close, to slightly darker than my hair, and do a strand test, and then do a "chunk" test if the strand test goes okay. That means dyeing a larger section of hair, and wearing it for a few weeks or even months, and see how it reacts. Does it feel really dry? Is it harder to comb than the rest of my hair (a sign it's been badly damaged)?, or am I seeing breakage or split ends? If that hair still seems glossy and happy, then I'd do my whole head (well, minus the test parts, anyway). And if my test chunk seems dry or damaged, I'd probably go with semi-permanent colors.

I know that wasn't a rock-solid answer, but I think this would be the safest thing to try with your hair. I hope that helps a bit!