I guess I'm lucky my FMIL is actually letting me and mom pick out what she is wearing to the wedding. She really does not have a fashion sense at all and would wear gym shorts and a shirt and clogs with socks on if she could. But she told us she doesn't want to look like she doesn't belond in the pics especially when she is standing next to my mom. As my mom likes to always me dressed up.
I completely agree with the other ladies to let it go. Be happy that she will at least look beautiful in her gown however the color might not be what you wanted. There are much worse situations to really get mad at LOL