—What truth? —There is no light.
I love it when portraits have a warm/cold duality in the light, but it's tricky to find a location where the lights are naturally balanced and positioned to create this look. Easier is to find (or set up) a location where a warm ambient light (a lamp with a tungsten bulb, say) brings one part of the equation, and simply add a low-powered flash for the cold component.
But sometimes there isn't even a light to be found or "augmented", and simply throwing colors at your subject won't look natural—it will look faked, produced. Instead of faking it, you can suggest it by using various hints.
See? No light here.
Take this location, for example. We had been driving around, searching for a nice looking background in back alleys. What was supposed to be an overcast day (perfect for this) turned out to be a clear blue sky, so we had to find a spot where we were in the shadow of a building. I was looking for something gritty, with texture and personality, which included a door, window or garage frame to break the uniformity of a simple wall.
Another reason for including a door in the picture is that it is easy to believe, even without seeing it, that there is a light bulb installed above or next to the door. All you have to do is compose so that the [non-existent] light appears to be just out of the frame, throw a gridded light there (with a good amount of CTO) and presto, it looks like there was a bulb:
One light. (I later added more CTO to make the color more vivid.)
To complete the illusion, it has to look like the bulb is throwing light on your subject. Simply add another light (of the same color) coming from the position of the suggested light:
With two lights, it now looks believable.
Without the door frame, the warm spot on the wall would have looked out of place. And without this believable source of warm light, where would it be coming from to hit your subject? It's the combination that makes the whole thing work.
Keep in mind that this was shot in midday, so the ambient light, although diffused, was too strong to shoot at a wide aperture and make it look like an evening shot, so I added a few stops of ND in front of my lens. That's no problem for the studio lights I was using, since they pack enough power, but I had to rely on battery packs, since obviously there was no power outlet available.
In this case, the heavy-lifting is done by the flashes, but the cool ambient light acts as a fill. I added a large soft source (a 60" brolly) in the opposite direction as the key light on my subject.
After some tweaking of the lights, we shot some frames until I got the relaxed, natural expression I was looking for. I was framing horizontally while setting up the location to have a sense of how the whole area looked, but I chose to frame it vertically and closer for the final image, as it looks more intimate that way. The background contributes to the feel of the image, but the picture is not about the background.