I wouldn't mess with the color at all. It's such a pain in the butt. Think about how fast the hair on your legs grows. The hair on your head is growing faster. You'll be doing the roots all the time and it's a pain.
It's also your last year. What if you mess it all up and look goofy? It could happen and you sure don't want all your last pictures being the one with the goofy hairdo.
Maybe a different style would be better. You can go sit in a beauty shop and look through all their cool hair books to find a style you think would look good on you. Or if you find one in a magazine you like, take it with you. Maybe just go in there with no idea and talk to the beautician and get some ideas from her as she probably knows more about current trends.