I had my hair highlighted in first trimester (and since also) I asked my hairdresser before and she said it was fine. Not sure if this would be the same as an all over colour which touches your actual scalp. Worth just asking your hairdresser if your unsure (as long as you trust them - I have seen mine for years so I know that she would just say)
It is fine to colour your hair, the worry with doing anything like this in pregnancy is only the risk of you possibly feeling unwell with the fumes (which is unlikely) or sometimes colours reacting differently because of the hormones. Sometimes colours can come out slightly different than normal when pregnant, no risk to the baby though. I am a hairdresser and also worked through my first pregnancy in the salon which my midwife had no problems with other than to make sure I am able to sit down if I got tired!