Mines black and its filthy. The only thing I have noticed is using REJEX wax.
REJEX is not a wax though. IT was a protectant made for airplanes and its found its way over to the Porsche community as a wax type product. The vehicle has to be real clean to use it. Its NOT a cleaner wax. Its just a shine protectant type protectant. The use on aircraft was to keep dirt and bug splats down and easier to clean. You might try it and see if it works. I use it on all 3 cars- the 2 BMWs and the GT3. I dont like it to get a great shine but it does make everything easy to keep clean. Its only about $15. Google it. The only think I can think of.