Irises do not change color - not on a permanent basis, anyway. It is possible that herbicide drift can cause temporary changes in iris pigmentation (do you use RoundUp or have a nearby neighbor who does?). If that is what happened to your irises, they will probably return to normal color next year, assuming they are not subjected to herbicide again. I don't believe there are any nutritional factors that can completely change an iris's color pigment, though growing conditions can certainly influence color saturation. It wouldn't hurt to have your soil analyzed for any possible nutrient deficiencies. The amount of available sunlight and ambient temperature can also influence color intensity to some degree, though not to the point of turning one color into an entirely different one i.e. purple irises turning white.

There are, however, other possible explanations for these apparent flower color changes:

1. Irises often do not bloom every year. It is possible that your dark colored irises neglected to bloom this year, while lighter ones (perhaps left by a previous owner) within the same planting area did bloom.

2. If you dug your original irises from a bed where only purple irises were blooming, perhaps you assumed that you dug only purple irises. However, there may have been a number of other colors of irises in the original bed that weren't blooming at the time you dug but that did bloom later in your own garden.

3. If you dug and replanted a bed within the last several years, it's possible a tiny piece or pieces of a previous iris planting remained in the bed and has now matured to blooming size.

4. An animal or child may have uprooted a rhizome from elsewhere and dropped it into your garden without your knowledge, or a neighbor may have tossed unwanted rhizomes over the fence into your yard where they took root. It happens.

5. Certain cultivars are far more vigorous growers than others, and in closely planted beds, the more vigorous growers will almost inevitably choke out the less aggressive irises over time.

6. Your original irises may have been bee-pollinated within the last few years. The seeds from those crosses may have ripened, dropped, and germinated among the original plants, and now you have a bunch of new seedlings blooming within your original clumps. Iris seed does not grow true to the parent plant and may well have produced some new colors in your iris bed.

7. On extremely rare occasions an iris may "sport", meaning the iris may produce an offset that differs noticeably in appearance from the original plant. HONORABILE is one well-known example of a cultivar that has produced several sports in its long history. The sport offsets, however, do not change the color or appearance of the original plant.

The only way to get all of this sorted out is to place a tag around each bloomstalk to identify its color, then dig and divide all of the rhizomes 6-8 weeks after bloom, replanting each color separately about 2' apart so they won't crowd each other too soon. Remove any seed pods that may form before they have a chance to ripen to avoid a repeat of this problem in your garden.