It is entirely up to you. Some brides like the mothers to wear things that coordinate with the wedding colors, others request a particular style but its really up to you if you want to have that sort of discussion with them.
I only have to worry about my MIL but I am probably going to have her wear a dress that is one of the colors that we use in our color scheme.
My mom and MIL didn't wear the colors of my wedding. The color of their dresses went fit perfectly into my color scheme tho!
My mom and MIL wore colors that coordinated with my colors and it looks just great!
my colors are navy blue and white, w/ a splash of yellow. so I was thinking to have my mom in gold or something of the sort. I saw a emerald green that look really pretty too.
haven't thought much about my MIL though. Gotta think about that!