It is perfectly safe to colour your hair while pregnant. You can use permanent or semi permanent colours..... Colours these days are a lot more gentler than they were 20 years ago. I coloured my hair and worked with colours with my 1st pregnancy and he is/was fine.

Those 20 washes colours are fine too. All they do is add colour. You may just find they won't hold as long as normal.

As for finding chemicals in your hair, you will find that if you colour you hair. If someone is taking asprin you can find traces of it in there hair (not with the naked eye of course, but when hair is tested they found traces of it).

I say go for it if you want to colour you hair.