Hair dyes have been around for many years now and there doesn't appear to be any evidence to prove that there are any links to birth defects. However, there is no research to prove they are safe either.
If you are over 28 weeks pregnant some airlines will ask you for a letter from your doctor or midwife. Most airlines will not carry pregnant women after 36 weeks or 32 weeks if they are carrying twins.
Folic acid is taken before the first 12 weeks of pregnancy as it supports the development of the neural tube. After 12 weeks it is not harmful to take but the neural tube will have grown and so it will not benefit from it.