Perhaps a failed attempt at applying directional lighting on a model would be more like it.
One of the regular Old Timers is looking to get into Imperial Guard and wants to paint his army up so they are a bit different than your regular Imperial Guard paint scheme. Sounds like every Space Marine player I know... "But I want mine to be different."
It's a really cool idea, but I don't know if he'll be able to pull it off. If he does, it will look amazing on the tabletop. He wants the army to look as though it's night and only portions of the models are lit by things light flashlights, fire, spotlights, etc...
The three colors I used
My little bit of Object Source Lighting experience only goes so far. When I've done it, the object was usually on the model and the "glow" was not very bright and only illuminated a small portion of the immediate area. This might be a bit different.
I said I would paint up one guy to show him what he might look like if he were crossing an open battlefield at night. The moonlight to his back and the "flames" of war in front of him.
For the back, it was a dark-bluish grey that I used to highlight the top surfaces of the model. I figured this would simulate moon "light."
For the front, I used an orange to simulate the light from fire in front of him. It took a few passes to build it up and I used a brighter yellow in a couple places for some contrast and to show the different textures (reflective qualities) on the model.
And then a look at the side of the model where one color meets the other... almost.
Turns out, this isn't what he was looking for at all, but I think it may have helped him figure out how to get the results he does want. I think OSL is one of those things that can be tough to explain to people and they don't really know what it looks like for sure, BUT... they certainly know when it looks wrong to them.
With as many models as the average Imperial Guard army has, he should be an expert in this by the time he's done with all his troops. Hopefully he'll figure out how he's going to do it and he'll end up with a great looking force in the end.
Make sure to check out these posts as they might help:
How to do basic zenithal highlighting