If that doesn't work I might try the heat method.
The heat gun method works by drawing oils within the plastic to the surface. It will work, but it will be temporary. With repeated application, the plastic will become more and more brittle. Being that yours are green and rare, I suggest finding another solution. Back to black works, but wears off quickly. I soak mine (black) with a quality tire dressing from Surf City that last longer, but also wears off. Shy of having them painted, you will have to work to keep them looking good.