Air Tractor 502Bs are almost always yellow. I don't think I've ever seen but one or two any other color. They're pretty fun to fly too from what I'm told. A motorist seeing one doing crop spraying near the highway would probably get themselves all worked into a lather.
Another pretty common one you'll see is the Cessna A188B Ag Truck. They're a little smaller and not nearly as fast. They are noted for being pretty rugged and able to keep you safe if you happen to ride one in as long as you're strapped in correctly, wearing your helmet, and safety gear for the chemicals you're flying. They're supposedly a rugged little bird I'm told. If I remember correctly the last time I had crop spraying done he was flying a A188B.