A lot of plants vary between juvenile, first time flowers and later mature flowers, you can also get variation according to how much light they have had in different years. Especially the darker ones. Marked differences are unusual, though you'll often find a flower becoming pinker or greener after pollination and as the seeds form.

Cheers Greenmanplants

I have a plant that was very dark purple last year and now is much more pink this year. It is maybe 4 or 5 years old.
Mary

I have also seen two distinctly different colours on the same plant in the same season. All we could put it down to was a second seed perhaps laid dormant for an extra season, but was accidentally potted up with the first seedling plant. Its such an easy mistake to make when pricking out the seedlings!