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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.