Shel, tell them just what you just said there that you didn't expect it to be exactly the same but something similar you would have been happy with !!
That way they can't use the tactic of saying Well we can't do it the exact same.

... um, i'm gonna sound evil here but look more stressed then you feel when you are telling them and tell them because your unhappy with the colour result it has stressed you out heaps and that all you won't be able to get your hair done for a very long time as you have a baby on the way.

Hey, no ones gonna knock back a stressed out looking heavily pregnant woman

All the best, you never know you might to be pleasantly surprised with the outcome tomorrow, it might turn out like my situation when the hairdresser was very understanding and i was put at ease and i was so glad i went back to say i was unhappy with the result and in turn walked out not only looking great but feel great too