You can change the beam (if it's one of Prowler's locos) by going into the texture file for the beam and adjusting the hue in your favorite image editor to color it whatever you want to. Same goes with any locomotive with "Ocemy-style" lights.
If you're referring to the light cone that lights up the ground and objects at night and in tunnels...I really don't know if light_color will change it, but you can try. It will most certainly change the corona color, I have done this on most of my reskins to get a more "incandescent" look to them, but I haven't changed the color enough to see a difference in the light on the ground and on objects. Mine are more modern diesels, so they're not as yellow as Sniper's, but his color looks perfect for old steam locos.