Notice the example:
The top image is a Low Key image. There is a single light source, which creates strong shadows on part of the subject. The rest of the image is mostly black. The bottom image is not a Low Key image; it is simply an underexposed image.
Often but not always, Low Key images are black & white. That is because the play of light and shadow creates strong contrasts which are better shown when the distraction of color is removed. However, colored Low Key can also be spectacularly impressive, if done correctly.
Let's take the same image as the one above (top). Obviously, the setup has been such that it allows for the creation of a Low Key image (single Speedlight fired remotely from the right side of the camera). Straight from the camera it looked like that:
Although it's already quite good, there is a hint of texture on the background. Plus, we want to increase the effect of light and shadow contrast and remove even more of the existing texture on the shirt, cap, and face. In other words, we want to block the shadows even further.
Adding an adjustment Curves layer (see picture inset for settings), we get the following result:
This is obviously not acceptable. The colors are oversaturated and the appearance is not natural at all. The solution? Very simple: Adding another adjustment layer, this time a Hue/Saturation one, and reducing saturation by about 45 points, we get a significantly more natural-looking image:
To increase the impact, I add noise in the following manner (to avoid the presence of noise in the lovely black area) :
1) Create a new layer (press the new layer icon on the bottom of the layers palette) and select "Overlay" as the blending mode
2) Use the bucket tool to fill the image with 50% grey
3) Filters>Noise>Add Noise. Use "Distribution: Uniform" and tick "Monochromatic". The percentage is up to you, I used 15% on this one.