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According to what I have read, irises do not change color. Some flowers do change color, but the iris is not one of them. Their color is determined by their genes and chromosomes.
I have irises that have been growing in my yard for more than 30 years. I've never had one change color.
OTOH, I have had a few die out for various reasons, such as my being unable to divide them every 3 to 4 years. Iris need to be divided every 3 to 4 years, or they stop blooming and eventually die out.
It's possible that the other colors died out for one reason or another, and the dark purple variety just took over. JMO.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams