The only way iris can "revert" is by your not deadheading the spent blooms, allowing seed to form and drop into the garden. The seedlings may then crowd out the parent plant. Hidden in the parent plant may be unexpressed recessive genes for lghter colors or white. If you get, for example, two white genes paired up in a seedling, you may get white flowers in that seedling, even though the parent plant was , say, purple (heterozygous purple with a recessive white gene.) Recessive genes are called recessive, because they can seem to skip generations. They need to be paired with another gene of the same sort in order to be expressed.
That's probably an over simplification, because iris genetics are so multifactorial, but it explains how surprises occur.
