Welcome. This sounds strange. If I'm understanding you they bloom the right color and then fade to near white? I know a few varieties of tall bearded that do fade a bit, but most often even in hot sun that doesn't happen. Water Waltz is one that fades a lot. You can see the different colors in the photo below.
Do you happen to know the names of the irises you have?
I don't have a lot of Siberians, but never had one that faded. Siberians do like acid soil.
Quite often irises bought from box stores turn out to be the wrong cultivar. Were these irises you bought or got from friends?
I'd recommend that you get your soil tested, and then amend it as needed. Then order a few irises from a reputable dealer, some really dark colored ones and see what happens.
