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The three LIGHT primaries of red, blue and green are defined as primaries only when using the Additive Color System. If you shine three flashlights, one red, one blue and one green, on a wall, you will get a white light on the wall. This is only if you use an EQUAL amount of each color.
The opposite is also true. Newton proved this with his prism: When you take white light from the sun, flourescent light or tungsten, the white light will separate as a rainbow - with the light primary colors being red, green and blue.