Answer: I have heard of this color change happening to iris before, but I'm not sure what the cause is. A friend moved and took her iris with her, having the same color change happen. But in your case, you're still gardening in the same location. Perhaps more shade from growing trrees nearby or a cloudy spring? You could try to give the irises some phosphate fertilizer for general health, but I can't imagine why that would influence color change. Sorry I couldn't help further.
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