You know your FMIL best so I figure if you think that she did this on purpose than that is the way it is. For me, it was easy because my Mom and I took my FMIL shopping for her dress and I was able to see what she was purchasing before hand.
I have a similar situation where a guest asked if it was okay for her to wear an orange dress and her hubby to wear an orange shirt as it would match our wedding colors (pink and orange). I came right out and told her that her husband could not wear the orange shirt as that is what the best man will be wearing...I don't really care but I didn't want people to think that he was part of the wedding party! KWIM?
Anyway, I think you should tell her that she should exchange the dress for another color, however, 5 days before your wedding, that might not be possible. Just breathe and remember that people will be looking at you and your husband to be...not the clueless MIL that is camouflaged in with the wedding party!!! AND I agree with Tam, your Mom is going to stand out more than the MIL, so tell her to WORK IT GIRL!
Congrats and have a GREAT wedding!!!!