I don't think I have ever seen such wide difference in flower shading on the same plant in the same year. Quite spectacular!
Many lilies offer different renditions of color due to their environment, and temperature is likely a major factor, as Magnolialover indicates.
Some lilies (and other flowers, too) are known for their color change as the flower ages. For example, a flower opens pink and fades to white in a few days. This Shibori spirea is a good example:
All flowers open a deep pink color. As they age, they fade to white. So the white flowers are old, and the deeply colored ones are new.
Perhaps both of these phenomena are working together in your lily, Darkkitten. I don't know if there is any way to tell, unless you observe the blooms in coming years as well.
