That's a tough one Back to Rejection Central No Rump Shakin eh?
you were trying to buy her a drink and she dissed you?

Well, this is a toughy, see you were willing to give the woman something in exchange for, at most, 3 minutes of smalltalk and she thought that your appearance or your approach or something about you wasn't worth it. It's a hard fact to face. Try to ignore your initial instinct to say something like: "That's good, I figured it was a bad idea once I got a good look at you anyway." because it may just give her more stuff to giggle with her girlfriends about (and that's embarrasing even the 10th time).

The reccomended method: Say "Fine", order a drink for yourself and leave saying something mean about her hair. This tatic works well because she is pissed but reluctant bring it up with her drunk girlfriends for fear that you were right about the hair.