Welcome Mayodog2
I know an iris of another color could "take over" a planting, but it seems to me you would see two different-colored irises for at least a year or 2 until the more vigorous one took over. So I have wondered what the cause would be in the cases where a complete color change occurs in a year. And now that you mention it, it does seem that these cases are always purple to white or purple to yellow, not the reverse.
I read somewhere that the anthocyanin production in grapes was inhibited when grown in a medium high in ammonia, so there might be something to the fact that these irises are losing the purple color in alkaline soil.
I'd be interested to hear what you find.
