I can see where you're coming from but in the end, I'm sure when you're in the moment, the color of her dress isn't going to matter. You're going to be so focused on enjoying your day and becoming a wife, that these small things (that may seem large now), will turn out to be relatively small bumps.
Look on the bright side- at least she's not wearing white (that's what my stepmother had picked out for herself to wear to my wedding!).