We sometimes have customers tell us that their entire patch of Iris changed from its original color to white or yellow. Iris cannot change color. Usually, when a customer notices this unusual occurrence, what has happened is that the purple (or red, or pink, or blue, or yellow, or black, or whatever color) Iris has grown into another nearby patch and the one color has dominated (or crowded out) the other. When dividing Iris in the summer, it is an easy mistake to make to throw out one color and plant the other in the entire bed.
Another culprit is weed killer. The use of herbicides, such as Roundup, near the Iris can affect the plants and cause flowers to go whiteish and contort. If they recover from the exposure to these chemicals, the original color will return in subsequent bloom seasons.
A third possibility is that bees have cross pollinated and a seed pod has developed. The seeds from these crosses can drop into the bed and grow and mature over time. Then one bloom season, a new Iris is found blooming in your Iris patch.