It's colorless because for the moment, you're hydrated. Your body is going to use a lot of water to flush that alcohol out of your system for the next few hours, though. Make sure you're drinking a good bit of water after each beer!
In one of my nursing classes, my professor said that alcohol reduces your body's ability to absorb water. That's why, even though you are drinking tons of liquid that contains lots of water, your urine is practically clear, because it's mostly water.