I have had this happen when an overlooked bee pod, a seed pod pollinated by bees, comes to maturity and drops its seed among my prized irises. This spring I found a lovely bright yellow iris growing in the middle of a lavender clump. I gently dug about and lifted it out. It had three fans and was not attached to anything else so I think I got it all. Seeds that come to maturity without pampering are usually the hardiest irises and can overcome another variety if not kept out. One little iris seedling, a yellow and white, came up in a clump of brown/green irises. I liked it so much I have planted it so that I may watch it grow and decide if I want to name it.
The question of irises changing colors has been brought up often and I think since the color and form are genetic I don't think it is possible.
