Carl, some pool store's are so fixated on the "DPD standard" that they don't test for combined chlorine: if there's no free chlorine present, they will report no chlorine at all. OTO will show yellow if EITHER free chlorine OR combined chlorine is present. Some (usually bogus) treatment systems deliberately produce combined chlorine. In such cases, it's possible that OTO will show chlorine, but that a DPD-fixated pool store will not. Of course, if the store goes ahead and adds potassium iodide to the sample, and retests for TOTAL chlorine, then they will see approximately the same value that would be seen with OTO.

Still, if a pool's turning green, and the pH is in range, MORE chlorine is called for, till the algae stops.

Ben
PoolDoc