I did haircolor through all three pg's and bf each for a year - all with colored hair. I have an unreal amount of premature grey - not a few hairs, like almost half of my hair! I couldn't find anything the definately said it was bad. I gave up everything else bad - every single drop of caffeine, artifical sweetner, non-organic foods - you name it.
I know that might make me a bad mother, but so far so good with all of the kids - very smart. I would have been an emotional wreck if I had been forced to go grey and I definately wouldn't have had the second or third babies - I would be too wacked out on major anti-depressants to get pg again. It makes me feel great about myself, which makes me a better mother. I say GO FOR IT!!!