I had an idea for an experiment that I might do one day, if I have the time and the energy. Take two examples of the same yellow leaved cultivar (or two each of multiple cultivars), same size, same pots etc. Rig up some sort of rain shelter over one example, such that it did not cause any extra shade, and have the other nearby in the same sun exposure but open to the rain. Wait and observe. If the one exposed to rain was the only one that gets spots it would be proof enough for me. I can't really think of any other way for a simple gardener like myself to put this to the test. Whether I will actually get round to putting this into practice is another matter entirely...