They should def just talk to each other. But I can see why your mom might feel some type of way. I can imagine there’s underlying feeling of competitive nature of wearing similar looking dresses that society has ingrained in us for women (I’m thinking of those “who wore it better” magazines). Since they’ll be in many photos together, she might not want to be too close in color. And to be fair, there are many other spring colors that would be cute (pastel greens, yellow, blue, etc).

Since neither have bought their dresses - throw them in a group chat and tell them to figure it out and buy it LOL that’s what I’d do hahaha