The yellow and white flowers are invasive wild radish. They're more commonly pink and white near Sacramento, but farther north they seem to tend more toward yellow and white. Apparently they're a hybridizing mixture of Raphanus raphanistrum and Raphanus sativus, uniquely adapted to California. The more colorful variations come from the Raphanus sativus parent; Raphanus raphanistrum is usually white.
The rice fields are flooded intermittently throughout the year and sometimes burned during drier periods (although the burning is increasingly restricted by anti-smog laws). When the fields are flooded, they almost always have egrets or other water birds in them, although I didn't notice any birds in the part of this particular field that I could see from the road.