just.a.thought.:

However, while her MIL has the right to choose something she finds flattering, it would not have been inappropriate for the bride to request that the mothers wear similar colors that coordinated with the theme of the wedding.

But she didn't make that request.

just.a.thought.:

. . . , so of course its natural for the mothers to coordinate as well.

But they didn't.

just.a.thought.:

Before my wedding, I sent a swatch of fabric to my future MIL and my mom. I told them to choose whatever style they wanted, but that I would like it if were long and in that color. I offered to buy fabric for them if they needed me to.

But she didn't do that.

just.a.thought.:

The one thing I have to say in defense of the MIL is that the bride should have requested that the MIL choose an outfit within a certain color scheme months earlier (before an order was placed) so that the MIL didn't have to guess or think the bride didn't care.

It's your one defense of the MiL, but it's the only relevant point here. She DID NOT ask the MiL in advance, the dress is bought, and that's history. So the only question is: should she keep her mouth shut. Answer: Yes.