My usual response to such things these days: give a placid little smile to the person who’s freaking out, take a deep breath, and say, “serenity now…serenity now…” as if you’re walking them through a meditation session.
Stick to your guns, just don’t act like they’re guns. In other words, be firm, but totally courteous. It does nobody any good to up the ante on rudeness. I’d say remind her that she did say she didn’t care what you chose; if it seems at all appropriate make a joke out of the “wear beige and shut up” line; if necessary tell her the outfit can’t be returned. But don’t back down. That won’t do anybody any good, either. I think I’d wait until she came to me about it, though. If she doesn’t bring it up again you don’t have to, either. Maybe just let it die, if it will. Then maybe you can all just get through the day.
Is it possible to get in a throwaway line about how nobody will be looking at you anyway? Or let the bridegroom do that? 
You took the words right out of my mouth. I felt like I had to jump in and say that I just got married at 22 and didn’t pull that stuff (sure, I flipped out about the location not telling me that the base where the wedding was being held was also hosting a Lynyrd Skynyrd concert at the same time. I had to find out from my Mother-in-law who heard it on the radio the day after my contract signing with the site.)
Hijack aside, your outfit sounds wonderful, the bride is completely out of line, etc. Wear what you bought. Our moms both wore black dresses of their own choosing (my bridesmaids wore black too). The only thing I said was that it was going to be a decently formal evening wedding and black was welcome but all-white was not. They were free to choose purple or lime if they wanted.
Oh, and when you go to find your green hair dye, ask for Manic Panic.