Grant if I can make a suggestion, when you make your selections before altering hue, color, etc... Zoom in really really far in so that your selection is as perfect as possible. Its that kind of attention to detail that will yeild good results. For example, on the windshield of that blue mustang you can se a harsh straight line made at the top. This kinda thing takes LOTS of time but its worth it.
Not tryin to dog on your newly found skills, just tellin ya what has worked well for me. Good job, keep it up!