Iris are unable to change color. No ifs, ands, or buts. If you have been reading other posts you will have seen how people thought they had lost an iris for 10 years and then it appears. Or buying a potted iris which has an extra rhizome of another color in the pot that doesn't bloom until the following year. The mysteries are always explained. Somehow there were bits of rhizome of a yellow iris in your patch which was smothered by the aggressive purple.

Thank you for your reply... But they did change...... there is no way that I had any yellow at all in my old yard... I planted nothing but purple ten years ago.... and have had purple ever since... I dug up whole clumps of purple and planted them... they are now blooming yellow! I went from a very wet mild area to the high Desert.! I'm not buying that they can't change...@

You dont have to buy it..but I can tell you irises dont change colors..something else went on with them... but they didnt not change colors