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.

This type of variation appears more as a flush or increase in pigmentation than a total change in color. For example, I have a H. x ericsmithii that flowers year-round; in summer it has greenish-white blooms but during cold months distinctly pink-flushed flowers.