In order to use a specific value scale effectively, you need to be aware of the relationship between value and contrast. Value key and value contrast are not the same.
All keyed art uses a limited range of the value scale, and thus a limited range of contrast.
However, if your artwork stays too much in the pre-defined value range, it can quickly become boring.
As a rule of thumb, to make a low-key image look interesting, you need to use a few well-placed highlights and create some contrast.
A high-key image, on the other hand, comes to life through a range of dark shadows, and some well-chosen contrast.
Have a look at Caravaggio’s ‘The Denial of St. Peter‘ below as an example of a low-key painting. Most of the colors are from the dark-brownish range. There are only two small sources of light (most likely candles) that are not even directly visible to the viewer.
They provide just enough light to make the people’s faces discernible, reveal a bit more of their clothing and of the ongoing scene. These are the areas that command the viewer’s attention because they stand out from the dark background.
When comparing how these highlights contrast with the surrounding darks, you’ll notice that the contrast is higher in those areas that are closer to the light sources. Anything further away from the lights has lower value contrast.