Depending on the type of white, white can be either a recessive or dominant gene. So if your yellow had a hidden recessive white gene, and if it self pollinated, it could have dropped a seed that could have paired white genes so that the color white could be manifested in the child plant.
But I suspect that more likely there was a rhizome for a white iris mixed in with what you got. Underthe sun is correct that iris don't change colors.
