You should talk to your mom and explain why you want everyone in a color and then suggest you go shopping together. Take her to several stores -- some she has been to before and some she hasn’t.
Department stores offer personal shopping services at no additional charge. You may want to make an appointment so a stylist can help her find a dress in a color that you both agree on.
But take her concerns seriously. Remember that your wedding day is a big day for her too. She is playing hostess and should feel comfortable and beautiful.
If in the end she won’t budge, not even to gray or navy, then you might have to suck it up and let her wear black. But maybe the style of the dress will be something you can put your stamp of approval on.