Set Subject: Dominant Colour
The key colour is the main colour in an image. Often, the key colour in an image is that which is the most dominant. Allowing one colour to dominate can lead to a powerful image. This is stronger when a primary colour (red, blue or yellow) is the dominant colour.
The colour can be but does not have to be 100% of the image. Images can be created in natural light, filtered studio light, a filter on the lens or manipulated in Photoshop. These can also be called monochromatic..