They same way you should do anything – directly and honestly. Is it going to go well? No, of course not. This is the classic no-win situation. There is no doubt at all here – you are, to put it mildly, fucked. But at least you have a choice. You can pretend you like the shirt, begrudgingly wear it, and generally hose yourself over while being a miserable whining snot, or you can bite the bullet and hose yourself over while being a stand-up sort of guy. Take your pick.
However … if you are going to be a stand-up sort of guy you need to remember that means being fair, too. Don’t jump to any sort of hasty opinion. Try the damn thing on, and do it with an open mind. Ask her what she thinks it would go with, and try it on with that, too. Then put the whole mess away and try it on again the next day. Still don’t like it? Fine – tell her, explain why, and take your medicine. In the long run everyone will be happier. And no one can say that you didn’t give it a fair shake.