If you like the color of green on the seats, you can buff them and the gloss will return, occasionally you need to do little wet sanding with a fine grit to remove the oxidation, but try buffing first.

If you want a different color, or one to match the grey you plan to use in the future, then you need to paint them with a good automotive paint if you want it to look good and hold up for along time. There are other options for paint if you want to spend less, but the look won't be as good.