Quite a Mystery

I must confess to being somewhat baffled by the mystery concerning my irises. In a nutshell they have changed colour since last year, they have morphed from a vibrant purple into a rather subdued pink. The photographs below show the same plant but with 12 months time lapse. If anyone can explain why this might have happened I would be very interested.

The image on the left was taken last year and on the right a few days ago!