SELMA, Calif. — The van rattling down a field road stood out even in a cloud of dust, its paint job the colors of birthday cake frosting, Christmas tree lights, red-purple-yellow-blue confetti. A party van helps when trying to reach farmworkers in dark pandemic times. But Ricardo Castorena, 47, found that out by accident. He’d only been trying to get free gas when he made the deal with the radio station. In March, when the pandemic first closed bars and festivals, the sales manager of Radio Lazer KLUN-FM 103.1 feared the station would lose name recognition — a name announced in promos by a galloping telenovela voice backed with the sounds of a laser-gun battle.