This has indeed been a strange year for all of us. I have one stem of Lilium regale that went purple from top to bottom just after it finished blooming. It coincided with a time when I was not able to water and the soil baked dry in the hot sun. In my opinion, it is likely heat and drought stress. But I will wait until next season to be sure in case I do need to remove it. I know that this particular bulb was planted more shallowly than my others (5-6 inches deep as opposed to 8-9 inches for the rest) so it is not as "insulated" from temperature extremes.
Many plants will produce purplish anthocyanin pigments to help protect their foliage in brilliant, constant sunshine. Maybe these lilies are just stressed and overheating and trying to protect themselves?
