A mysterious man walks through a dusty alley. He holds a gun in his hand. His face is concealed from the audience by scraps of cloth. Who is he, where is he going and what does he intend to do with the gun?
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On of the roles of the composition of an image is to depict the most important elements of the picture in a clear, recognizable manner. For color pictures, not only the placement of the elements is playing a central role, but the color composition as well.
An obvious method is to separate diffrent elements or layers within the image like foreground and background by different color saturations of a difference in lightness.
There are many approaches for setting up a color composition. To acquire some helpful rules of thumb, it can be useful to look at some of the oldest works of visual design in history: Coat of arms and flags.