Iris cannot change colour. Usually what occurs is they grow into one another and get mixed then the wrong plant is replanted. If you use weed killers such as Roundup near Iris, it can affect plants and cause flowers to go whiteish and contort. If they recover, the colour will return. If the soil is depleted of nutrients then the flowers may become a paler version of themselves. This will improve once they are replanted and/or fertilized.