I am playing with an image in which I've placed a vehicle on a new background. I've applied a filter to the vehicle to get the colour balance close to that of the background.

There is now some lighting from behind the vehicle which (were it a real photo) would result in a halo or edge highlights along the top of the vehicle.

I don't really want to post the image yet, but what's the best way of adding a 'fake' halo in this manner.

(There are no existing highlights on the vehicle image to enhance)

Hope this makes sense.