I've grown geraniums for years. I take some of mine in every fall and let them grow under reduced time under my lights. I then take cuttings in the spring.

I have noticed that as geraniums get older those marking tend to fade. Even the ones with just a little difference in leaf color eventually turn solid green.

There is really nothing you can do to bring back the leaf color. This doesn't bother me because I grow them for the flowers not the leaves.

If you really want to keep the color on other plants you might consider starting some from seed. They are really easy to grow from seed but do take about 3 months to get to the flowering stage.