I died my hair at like 4 weeks, it was perfectly fine. Died it again a week ago, and I'm now 18 weeks. It's perfectly fine! But the different hormones in your body can cause your desired hair color to not be right. That's the only downfall!
The whole "can't color your hair while pregnant" thing is a TOTAL myth. I'm a cosmetologist and I can promise you it will literally do nothing to your baby. First trimester or not. I would say that sometimes skin can change during pregnancy and can sometimes cause an allergic reaction to color, so try a patch test.
I've dyed and relaxed my hair my whole pregnancy and my seed is A ok... go head girl and beautify yourself..