You have a couple options available to you:
There's a tutorial called, "Replace colors," in the Tutorials folder when you first open PS Touch. Follow that and then apply what you learned to your car image.
You also have the option of using an FX brush. (Tap the active tool in the upper left, swipe down and hold over the brush icon; a drawer should pop out where you can select the brush fx icon.) Tap the option below the brush option to select a desired FX (color balance is listed here, the same effect used in the PS Touch tutorial). You would use this brush in just about the same fashion as you learned to replace colors in the "replace colors" tutorial, even down to selecting an area using any of the selection tools before painting.