Simple solutionfor the bride to address so-called MIL dress problem---bride needs to get a lovely shawl in a coordinating color with her bridal colors and goes with the MIL's dress's blue, and give it to her MIL as a gift--saying that you wanted to give her a present and when you saw the shawl, you knew it would go perfect with the dress for the wedding.  That will encourage the MIL to use the shawl for the wedding, making her more modest and color coordinated (for the bride's peace of mind).  If she doesn't use it--well, you tried.  Never hurts to give your future MIL a present too!  Otherwise, let it go!!!  In 3 months post wedding you will NOT care!!!
---married almost 3 yrs

I agree, this would be a great solution, as would the bolero suggested in another thread by the seamstress.

But I think the bride may have a bigger problem now - if her FMIL reads Dear Prudence, the cat is out of the bag! Pretty hard not to recognize the scenario, even with the plethora of sage/brown weddings....