I dyed my hair at 12 weeks. I asked my doctor and midwife and consulted my pregnancy book, and they all said it was safe. The doctor and midwife said it was completely safe but if I felt more comfortable, I could wait until im was 12 weeks - which is what I did.
Apparently the amount of chemicals that can get into your bloodstream fro hair dye is so small it won't harm the baby, and if you get highlights/lowlights, then there's even less "risk" as the dye doesn't touch the scalp at all.
I get alot of greys, so i'll probably end up dying my hair twice again during my pregnancy - so three times in total, as opposed to once a month which is what I did before.
Hope this helps x