Thane, I'd be a bit suspect :-)) Flowering plants generally do not change their flower color. There are some exceptions - some cultivars will revert to species but they tend to be only one or two steps genetically removed from the species and various stressors can contribute to the reversion. Modern selection of Helleborus x hybridus are so heavily hybridized with so many genetic inputs it is difficult to identify clearly with any species parentage.

When you see a dramatic change in flower color, it is most likely that seedlings or offspring have grown up and replaced the original plant. With the heavily hybridized hellebores, the seedlings could (and most often will) offer a very different appearance. Otherwise, a weak seed strain or just inferior hybridizing could result in flowers that are rather lackluster in appearance - there's a lot of them out there, unfortunately. Cultural factors could come into play as well - excessive sun can bleach out colors, soil pH may play a small role, excessive or improper fertilization, etc. You get the picture :-))

FWIW, I have mature clumps of x hybridus that have been in the ground for going on 20 years that have maintained their color exactly - the "blacks" remain black, slates continue to be slates, apricots are apricot and even spotted and picotees selections produce consistent flower colors year after year. If you buy quality plants from known hybridizers or recognized seed strains and grow them under proper conditions, the colors should stay true regardless of the age of the plant.

And they will self-sow, at least the h. hybrids and h. nigers. I usually leave the volunteers in place, but it's probably better to move them to a nursery bed and see what they turn into.

My Hellebores have been white for years and this year they are dark pink. Not every bloom, but a lot of pink mixed in with the white.

Almost all H. niger get a pinkish or pink tinge at sometime in the life of the flower. As mentioned temps and light or even just aging can cause it. A number of the other shades often take on a greenish tinge as they age, but I can't see that there is a year to year consistent change.

Since xhybridus have such a broad genetic range and self-seed prolifically, seedling plants often pop up as pink regardless of the color of the mother plant. That is their genetic default color.

I have definitely had this experience with one of my helibores. My mother in-law mentioned it years ago and I somewhat scoffingly replied with a very similar comment to the first from gardengal48 ...but then it happened to me!

I bought 2 very dark purple/slate/black helibores, gave one to her, kept the other. They are both in pots, flowered again beautifully the 2nd year, but then the 3rd year and this year, every single flower on my plant is greenish/faded pink. My mother in law's plant has kept on flowering in the original colour!

Hers has hardly grown (it's pretty neglected in its pot), mine is 4 times the size but changed colour. It's definitely the same plant, not offspring. Mine possibly gets more light (but only when it's in flower, the rest of the time it lives round the north side of the house), other than that, I can only imagine it must be to do with soil conditions.

I am going to re-pot it and see what happens next year...