Irises do NOT change color, just as you cannot change color. Two things can happen, a bee pollinates a flower from a different color, the pod ripens and the seeds fall to the ground and grow and then bloom later in a year or two. So then you have a flower of a different color in the same place.
The other possibility is that a stronger growing variety took over and grew across a weaker variety, thus choking the weaker one out. My irises grow across each other all the time, they spread out all over the place. This is probably what happened.
Dot
