Q. What should my mother or future mother-in-law wear to my wedding?
A. This is a matter of preference. If you have a strong opinion make sure you let them know how you feel. Communicate with your mother and future mother-in-law about what you would or wouldn't like to see them wear that day. Take into account the formality of your wedding, you don't want the mothers to be over-dressed or under-dressed compared to your wedding party.
According to etiquette, mothers should not wear the same color dress as the bridesmaids. Or the bride - of course!
In my case, I asked my mother and future mother-in-law to please not wear black (the color my bridesmaids are wearing) or red (with everyone in black and white, the pop of red in the wedding pictures would have drawn your eye right to that red dress!).
Ask the mothers if you can go shopping with them. They will be happy to have your approval on the dresses. It was a wonderful experience to go shopping with my mom for her dress - she got to be the star of the show and I got to tell her how beautiful she is for a change.
Encourage both women to talk to each other about what they are wearing. Your mom may remember something you asked that your future mother-in-law does not! ;)