White capri pants for a guy - what type of underwear?

My gf has bought me a pair of white capri pants. They are cute and I like them. The only thing that bothers me is that they are rather thin. My gf wants me to wear them tomorrow to campus wearing white thong undies. I just tried it one and you can see the outline of the thong through the pants. I told her, and she said not to worry, that no one will notice. Should I wear a different color thong or what? I mean, if I wear briefs you will see the underwear lines, so a thong is better. Maybe just a different color?