Well... not my iris but that question came at our garden club meeting last night and I needed to do a little research.
Some thought the colors might exist already but did not show because of overcrowding, while others mused that the change in soil might have brought on the change.
Soil change is the one thing that seems to be discounted, but a good response I found in a "Garden Web" blog from seven years ago seemed to have some of the best possibilities. (None are apparently confirmed)
Our club member mentioned that her iris roots had been dug up for a yard project and had not been replanted for several weeks.
Stress seems to play a big part in the process, somehow causing the production of certain pigments in the flower tissue to be suppressed for a time.
With a lack of the "good" pigments, the background color of the bloom is all we can see (and it was there all along, but overlaid with another shade) so the blooms appear to have "reverted" to, say, a ruddy gray-blue Iris instead of the glowing peach flower we were expecting.
Once the stress factors that have caused this to happen go away, the plants can actually produce their normal flowers once again.
An overgrown and starving clump of Bearded Iris which has "mutated" might well smarten up in a year or two after being divided and moved to a site with richer soil.
Heat and cold can be short-term factors in causing perennial flowers to change. During periods of extreme heat, for instance certain flower pigments seem to break down quickly.
A blossom might open in a soft lavender shade, but within a day or two it fades to more of a ghostly white. Once the weather cools down the color is retained much better -- that is, if the plant is still blooming.
Of course, self-sown seedlings, the more common "mutation", will sometimes appear in the garden and the seedlings are likely to produce flowers in shades other than the clear pink (or whatever) we see in the mother plant.
If you allow the seedheads to develop, there is a good chance self sown offspring will appear right within the parent clump. After a few years it would then appear to have reverted.
The original "mother" plant may, in fact, be just fine, although now terribly mixed up with its own progeny. Often, though, it is out-competed by its own more vigorous progeny and it fades completely from the garden.
True genetic mutations do, indeed, occur from time to time. These plants are called "sports" and are a common way new plant cultivars are discovered.
Many a great new perennial has appeared spontaneously in exactly this same way, seemingly by accident rather than the result of an intentional breeding or selection program.
Personally, I would like some new colors, so I may test this theory by digging some our iris, stressing them and planting them again. They did not do this when I divided them a few years ago, but I did not stress them more than a few hours.