I would address it in such a way that you're not disagreeing with her or giving her anything to be able to get angry about: I was envisioning you in blue which would match the corsage I ordered for you! What if you went with yellow? I was hoping you would match my Mom. She's wearing coral. You in? Seeing as how you don't look a day over 35 you'll confuse people as to who's getting married. *followed by big cheezy grin* If you wear white it might fade into the picture and we want to make sure and capture your presence at our special day. How about teal? ***The key to egotiating something like this is to give them a easy answer as to why not white but then immediatly followed up with an alternate suggestion. Then the argument isn't over the white because you're placing a new question in the way. Shut down the discussion on white, side track with a suggestion on a new color, aim for an immediate agreement (i.e. no more discussing the white)