It honestly doesn't matter what color you dye your hair starting the second trimester on. It all depends on the fumes, especially in bleach and ammonia for blonde hair color. Open all the windows or be in a well ventilated area during the process. A weird side note, I had dyed my hair with blonde highlights the day before I found out I was 5 weeks pregnant and it was one of the most painful experiences I've ever had due to the sensitivity. Keep in mind that it's not uncommon to become more sensitive to topical items when pregnant.
Just make sure your in a ventilated area to reduce the fumes
My doc told me the best time to color was in the 2nd trimester.