Ill have to be in the minority I guess...
Just wear the pastel bowtie, she might have picked it out to match her own attire for the night.
All of the posters above me are looking at it as a control issue, it's simply a relationship one and a silly one at that.
Ask her why she wants you in that particular bowtie, if its a good enough reason then consent. If its not, tell her you don't like it.
You can't have a happy relationship without knowing which battles to pick... This isn't one of them.
I think there's a double standard there, and it's not really a good one. If he told her to wear a summer dress do this winter occasion, how do you think she would react?