Red, yellow, or green will indicate the levels of diesel or exhaust in the air, while creatively illuminating the scene at a nighttime gathering. If your balloons are mostly red, I'd suggest relocating to a less polluted locale.

If you have some basic electronics/soldering skills and the ability to purchase a Figaro Air Quality Sensor, an 8 pin PICAXE microcontroller and DIP socket, a rechargeable lipo battery, a 10K or 100K resistor, and a tri-colored LED, you can build one of these guys yourself. You can find the exact directions to recreate these balloons over at Instructables. [Instructables via BoingBoing]