Step 11: Going back to the mouth, make sure you have the brush pallet open (Press F5) and select the ereaser tool. pick a fuzzy brush (see below) thats about 10% the size of the mouth, start ereasing around your mouth to remove all hard edges, make sure you dont erease any of the actual mouth.
Step 12: Set the opacity of the mouth back up to 100% and using progresively smaller fuzzy brushes erease the rest of the color that covers the fog lights, grill, under the bumper, headlights. basicly everywhere that isnt solid paint (Red)
Step 13: Go to Image>Adjustments>hue and saturation and change the color of the mouth to the color of your car, If you have a black white or silver car dont just change the saturation to zero! there are about a million reflected colors in your paint, look at whats most domminant (usualy a light blue) and keep a little of this color in your mouth.
Step 14: youll notice that you can still see some of the grill under the mouth, the best way I've found for getting rid of this is tedious. going back to your car photo and using the healing and clone stamp tools. If you arent comfortable with these then using another fuzzy brush and painting in some of the color of your car over the hole will also work this takes a pretty long time, I usualy try to clone the piece of the bumber between the fog lights and grill and use vertical strokes to cover the middle. heres an in process shot:
Step 15: Go back up to the eyes and make a general selection around the eyeballs, then feather it out (like before) about 5 pixles, Then select the inverese again (ctrl+shift+I)
Step 16: Do the same color adjustments as on the mouth and then you should be done!
There is plenty more to add, you can use the wheels or just about any aspect off the cars wallpapers, Play with it! then post what you make!