To put a different view :

I did not dye my hair during either pregnancy. My DH feels strongly that it should not be done. Personally I waver - there are arguments that unnecessary toxins absorbed trhu the skin during pg can have a damaging effect on the foetus (for example there are quite a few studies that link excessive makeup use to problems later on). I wear next to no makeup and try to limit chemical exposure so wasn't really fussed by the "risks" however real or small or whatever) but it was something DH felt strongly about so I didn't do it. I figured they were his kids too so he had a say.

Now I am using permanents all the time (the greys are a little stronger these days LOL!). What will I do when pg with number 3? I honestly don't know. I suspect DH's stong stance against it will lead be to use foils (no contact with scalp) or a natural dye. There is also that gut feeling I have that I don't want to take the risk.

So my advice is to do some research and weigh up the risks for yourself. But I would not take the advice of your hairdresser seriously - they are not experts in the area and a sample size of a few 100 (or event 1000) preganant clients with "no problems from hair dye" is not statistically sufficient for them to make a call. Plus they have a vested interest in continuing to use whatever dye they are currently using so are not entirely unbiased.

IMO this is just one of the many risks you will have to weigh up as a mum, so welcome to the world of worrying about your baby LOL!!