Answer: Well, hybrid irises don't really change colors but lots can happen to make an iris bed look different after a few years. Certain cultivars are far more vigorous growers than others, and in closely planted beds, the more vigorous growers will overshadow and eventually choke out the less aggressive irises.
Your original irises may have been pollinated and the seeds from those crosses may have ripened, dropped, and germinated among the original plants, and now you have a bunch of new plants blooming. Iris seed does not grow true to the parent plant and may produce new colors in your iris bed ? usually a yellow or purple.
If you allow the seedheads to develop, self sown offspring will appear within the parent clump and overshadow the parent. After a few years the clump would then appear to have reverted. The original "mother" plant may, in fact, still be there, although now terribly mixed up with its own progeny which will likely choke it out.
On rare occasions an iris may "sport," meaning the iris may produce an offset that differs in appearance from the original plant. The sport offsets, however, do not change the color or appearance of the original plant, only the new plants.
Purple and yellow are two colors modern iris descended from and tend to be the most vigorous growers, but also can be colors that other varieties "revert" to over generations. About the only way to keep your new purple and white iris true is to plant it by itself so you can watch it closely and remove any seedheads it may develop.
Best wishes with your irises!
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