When gerbera flowers turn green it means they have a viral disease and must be dug up and destroyed as it can spread to other plants. There is no cure.
Gerbera Daisy don't change color but often a different colored plant was potted together and will suddenly make a blooming appearance and through off a gardener.
It is unusual for it to take this 18 years to show itself, but not unheard of.
Congratulations on this long lived plant.