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Question: If red, orange, yellow, green, blue, indigo and violet are the colors of white light, then how come when you mix the colors together you get black?
When white light passes through a prism it indeed splits to reveal the colors red, orange, yellow, green, blue, indigo, and violet. Each color represents a different wavelength range of the visible spectrum. This sequence of colors is usually recalled, in the USA, by the amusing name ROY G BIV, or, in the UK, where I am originally from, by an interesting piece of history: Richard Of York Gave Battle In Vain.
If one recombines these different light wavelengths, it actually does just regenerate white light again. So why does one get black if you take these seven colors in the form of paints and mix them? The answer lies in the nature of light absorption and why substances have the colors that they do.
To understand why different colors exist, recognize first that the visible light spectrum is dominated by the primary colors red, green and blue. Mixing all three produces white, but mixing any two of them results in different colors, called the secondary colors. For instance, mixing red and green produces yellow, mixing green and blue produces cyan, and mixing red and blue produces magenta. Other colors are formed when you combine secondary colors with each other or with the primary colors.
Colors can also be obtained by subtracting one or more components of the visible spectrum, for example, removing both red and green would leave blue, but removing only blue would leave both red and green which combine to form yellow. It is this subtraction process which results in the color of substances, with different wavelengths of the visible spectrum being absorbed by the molecules in the material. The color that we observe is essentially the light that is not absorbed, that is, the light that remains and which is able to pass through the substance or reflect from its surface.
Paints have chemical compounds in them, called pigments that strongly absorb specific regions of the visible spectrum to yield the color of the remaining (unabsorbed) wavelengths. A red pigment would absorb green and blue, a green one would absorb red and blue, and a blue one would absorb red and green. Consequently, if you were to mix paints with all the colors of the visible spectrum, you would create a composition that was able to absorb all the wavelengths in the visible region. Such a mixture would not allow any light to pass through (or reflect off its surface) and would appear black
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