i posted this awhile ago, but i'll repost it now theres a graphic tutorial forum
anyway, step 1: find a pic of a car
step 2: create a new layer:
step3: select a color that's easy to see, in this case it's bright pink.
step 4: in your new layer, press alt + backspace and lower the opacity to about 50%.
step 5: Use the polygonal lasso tool(or whichever lasso you find easiest to use) to clear out the areas you don't want to color(in this case, the white bg, the cars lights,wheels,license plate and interior).
step 6: Hold down ctrl and click on the layer you just worked on.
step 7: While selected, switch to the layer with the image of the car.
step 8: Goto Image>Adjustments>Hue/Saturation and pick a dark red color.
step 9: Now right click on the layer with the pink cuttings(in this case) and click on blending options. Tick the gradient overlay box and apply these settings:
(You may modify the angle to get different results.)
step 10: Now change the opacity, anywhere between 30 and 40% would look ok.
There you have it..