It is an additive colour mode. It is known as additive because of the black colour of the background screen. It helps in developing and displaying pictures by adding light in different variation to the black colour in the background. The editor can clearly see the difference between original and changed image.
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It is a subtractive colour mode which is used for printing pictures on white paper by subtracting the ink. Furthermore, it uses secondary colours also which are quite lighter as compared to those which are being used in RGB. If you intend to make your colours darker, you will have to add more ink.
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