I looked this up in detail and found hugely contrasting information but nothing on any trusted ''medical" sites apart from what Flickan says...make sure the place is well ventilated.
I didn't colour my hair through my pregnancy. In the last few weeks I looked for someone to come to my house but I ended up being too tired to be bothered!! Have to sort that out soon as I recover enough from my c-section. I really wish I had made the effort as I know I look a mess in the photos and I'm usually fussy, particularly about my hair! My advice is get it done early so you'll be ready for post birth photos and still be able to rest up in the last few weeks :)
I do high and low lights ...i just waited until 2nd trimester
As long as it's well ventilated, your fine. I did with my daughter and no issues. Coloring through this one too. Have to...can't have the grey show ;)