Irises do not change color. The light colored iris you have blooming are different plants than those that bloomed purple. They may be seedlings from the purple iris if you did not remove the seedpods formed after the purple iris bloomed.

Treeman is correct. Iris do not change color. Weather acounts for richer/paler colors.

Another possibility is there were some small pieces of another iris' rhizome in the soil - could be from years ago - and these have finally grown to blooming size. Or you received another cultivar mixed in with your purples. Seedlings generally take 2 years to bloom.

Your purples produced baby rhizomes (I hope) that have not become large enuf to bloom this year.