Question regarding adding marker lights to an MTH GP40 (20-20840-1)

Replacing the lights to change the color should be fairly simple, but hard to say without actually seeing it personally.  All the lighting is LED, so replacing the physical LED is simple, I don't know if it's just the LED color or you have a lens that you'd have to replace as well.

There are a couple of issues in play here for powering extra lighting.  One is whether the board supports the lights, but the second is if they would be activated by any available chain and sound files.  You can see if there are available lighting outputs on the board from the diagram below, but I have no idea if you can activate them.  I'd connect the board to my test set and load files to test the funciton if I were trying to do this.

From the diagram, it appears that you have separate front and rear ditch light outputs, beacon output, markers, number boards, MARS front and rear, as well as headlights and cab lights.  It does appear that the lighting outputs are there, the question is, do they do anything.

Click on the diagram for a full sized version.

If this is production PS-3 would not surprise me that the connectors are in the engine wrapped up unused.  If so you can add markers and see if they work.  G