Next May I am going white water rafting near the Grand Canyon, on the Colorado river. How cold will it be? Should I take anything special with me?

I have never been on the Colorado River, but I have gone rafting. Wear clothes that can get wet but dry quickly so you aren't stuck in heavy, wet clothes for an extended period of time. I wear cheap Ked-like sneakers that I don't care if they get destroyed. You need to have some traction on the bottom of them, so don't just think old sneakers. Wear waterproof sunscreen. If you are doing the paddling, you might want to consider gloves that help you grip wet paddles and keep you from getting blisters/calluses.
As for the weather, the SW tends to be cool in the morning and night and warm/hot in the day. Therefore, dress in layers.
Bring some ThermaHeat packs and ibuprofen for afterwards. (I always fall in the water at some point.)

always cold