Color theory is crucial to color photography. Clever use of color can create harmony or drama in your images. Check out this great video to learn more about ways you can combine color in your photography.
Coming to you from John Gress, this educational video discusses and explains the six most common ways to combine color. Each one of those can be used to create harmony, unity, or perhaps separate your subject from the background.
A useful tool that helps with combining color is the Color Harmony Calculator. It can be used to create different palettes. If you're a portrait photographer, this will come in handy when deciding what background to use, and if you're a landscape photographer, this can be handy when shooting vibrant scenes.
Color theory is used extensively in movie productions too. Depending on the scene, the clothes of the main character may contrast with those ones of his enemy. Perhaps during a romantic moment, the palette will be on the warmer side. Fight scenes, such as the ones in Game of Thrones are often shot in colder tones. Wes Anderson is also an obvious example of someone that uses color theory extensively to create his movies.