Change of colour

This leaf caught my eye down the garden the plant has usually only green leaves but for some reason this year a group of them have turned this vivid shade. It has been here since we moved in over 30years ago and I think belongs to the knotweed family.
The giant knotweed is a pest that has roots that go under foundations and are impossible to get rid of this thankfully this is a much smaller & tamer version and doesn't spread.
Hope it's not mutating into it's rogue cousin!