first duplicate the layer of the car [right click on layer in bottom right and click duplicate layer]
use the polygonal lasso tool [on the left hand side where the tools are, right click on the lasso tool and change it to that one]
go round the car shape [on the new duplicated layer] missing out the wheels.
then use the eyedropper tool to get the colour you want from another image or whatever
then fill the selection on the new layer [alt + del]
then change the opacity of this new layer thats filled in [bottom right hand corner] to less than 100% so you can see through it.
then remove certain areas like windows etc as you dont want them to be filled in with colour
it should look like this if you hide the original layer

once its how you want it [just the colour left] change the layer style [the thing that says "normal" in bottom right] to a different type, i used multiply, change the opacity back to 100%
done
