You've received lots of good advice here, gradfashionista. Like all advice, our responses come from our own personal experiences and values, which may or, may not, be comparable to your situation and values. Finding a solution that works for all parties is probably the ideal but, in my experience, rarely happens unless all parties are willing to re-think and modify their original position. It sounds like you are trying that route, but not getting much help from the others--namely, your future in-laws and SO.
What I've found works best for me when I'm faced with a similar situation is to try to move past my position and empathize with the other person's side. Sometimes my assumptions about the other parties' motives are correct but, more often then not, there are other issues at play that warrant my consideration before I decide on my course of action.
North American culture sees a wedding as a primarily social event that celebrates a contract between two individuals. Viewed in that light, the bride and groom ought to do what makes them happy since the rest of the participants are supporting players whose function is to celebrate with the couple in the chosen way. But, for many other cultures and generations, weddings are viewed quite differently. The bride and groom may be seen as bit players in what is essentially a blending of , or contract between, families. Or the event is viewed as primarily a family celebration. Or the bride is being formally being turned into a member of the groom's family. Or the wedding is primarily a religious ceremony rather than a social occasion--or any number of other combinations and permutations.
If you can figure out why the color and style of your dress is so important to your future MIL, you'll be in a better position to decide on a course of action. If she is just being overbearing and bossy, that calls for one response, but if she is concerned about your acceptance into your SO's family circle, then that might call for a different approach.
It was my students who taught me that my preliminary assessments of their motives and reasons behind some of our conflicts could be dead accurate--or way off target. Once I learned more about the conflict from another's perspective, added a little empathy for their situation, and then decided what to do, I was much happier with my final decision. It also seemed easier for my students to accept a decision that went against them if they realized I had taken the time to understand their position.
