I installed a testable RP valve recently and now they tell me that the brass has turned white. It's in a chemical plant. Do you know why?

Yes - It happens especially quick when around chemicals. Brass is effected by the surrounding air and often turns white. Similar to how copper turns green. The environment around the backflow preventer is what's causing your issue. Sometimes it takes years, sometimes it happens quickly, sometimes it doesn't happen at all. This issue should not effect the operation of your assembly.