I really wish that the physical sky was implemented properly because everyday someone will run into this problem. When the physical sky is created, it attaches a lens shader to the camera to control the exposure(else you'll see exactly how bright the sun is) and that lens shader also has a gamma property and that is set to 2.2 which is the default gamma on monitors (the physical sky setup assumes you are working in a space where the gamma is 1.0 also known as linear space).

The thing is, when you choose colors from the color picker or use or your jpeg and other 8 bit textures they already have a gamma of 2.2 embedded (or else they won't look normal and pretty to human eyes), so now, you have your colors and textures being hit twice by a gamma of 2.2 which causes the washed out look. You can fix this by countering the embedded gamma, plug in gamma utilities(as you would with any other texture) into your shader(excluding bump, normal and displacement maps) then you can use the gamma node's value to plug the textures or change the color. The gamma node's "gamma" should be set to the inverse of 2.2 which is .45454545454 or you can round that to .455

Apart from that, I think part of your problem is the fact that your car is highly and equally reflective regardless of viewing angle so it's showing a lot of everything else but it's diffuse color and car paint doesn't do that. If you were to browse the mental ray materials section of the hypershade you should see carpaint shaders.