Traditionally, the mother of the bride purchases her dress first and then alerts the mother of the groom with a friendly fashion update. The information you share will guide the mother of the groom in her decision-making process, subtly ensuring her colors won't clash with yours and she won't make a selection that's too formal (or not formal enough).
Really, though, this tradition is completely dependent upon your sensibilities. If you're not a hard-and-fast follower of traditional etiquette, you may want to take a different tack. Invite the mother of the groom to shop with you or give her free reign to find a dress of her own liking. Building this healthy communication early on will pave the way for two families to come together -- and will certainly come in handy down the line when holiday gatherings or new babies are afoot.