Step 1

Straight to the point, first off you can pick any car, but in this case I picked a Mercedes...
Ok first you get any pic an open it with photoshop...

Step 2

Ok now change you brush setting to match the size of the headlight.

Step 3

in this case the bush size is 45, also make it 0% hardness...
After you do that go and pic a color of your choice, (but it has to be light for best look)
in this we choose a light blue color (C5DCFF)
now after you have chosen the color and brush size you go to "Mode" (under filter)
and click "Linear Light"

Step 4

now you go right over the headlight an click once. It should look like this.