The genetics in a rhizome is set from its beginning. It may be paler some years or a darker hue in others but the color of the bloom cannot change. Once it blooms the bees, and other insects, go from bloom to bloom collecting the pollen. The iris will then set a seed pod. If you are not diligent in cutting each and every seed pod, often called a bee pod if the insects did the pollinating, they will mature, pop open in summer, and scatter the seeds among the iris clump. No irises seeds will be true to the parent plants. The seeds will be a mixture of all of the genetics that your original iris came from. Each seed in a seed pod will be different. Some of these seeds will be fragile and not be able to thrive in an established clump but some will be hardy and if they are more hardy than the parent plant they will take over and your parent iris will be crowded out. How quickly this occurs depends on the tenacity of the parent plant and that of the seedling.
It is possible that you had a white seedling among the original clump you dug and it had not matured enough to bloom. It is possible that a seedling from a bee pod has grown in your established clump.
When I was an active Master Gardener for my state one of the most frequent statements I heard from gardeners was "do not plant yellow and white irises near each other as they will all turn white". The gardeners learned this from long experience. It was very hard to convince them of the habits of the iris. The truth is an irises genetics are set from the time the seed is created and cannot change.
I hope this rambling explanation helps Dewayne.
