A color is called primary if it is not obtained by mixing other colors unlike secondary colors, tertiary or different shades that exist.
The primary colors are the basis of all the colors.
The diversity of shades depends of mixtures and various determinations carried out.
There are two color schemes: additive synthesis and subtractive synthesis .
Each system has three defining PRIMARY COLORS. The choice of one of these systems depends on the use we want to make of the color.
The color principle most often taken in books or websites, although it does not specify in general the difference between the two systems is that of the subtractive synthesis; the latter considering color as a material (paint, ink, powder ...).
They will talk of additive synthesis if it is to use color through light (projectors, TV ...) and subtractive if the use of color is material (paint, ink, powder pigments ...).
For additive synthesis The primary colors are red, green and blue : RGB (Red, Green, Blue ) .
For subtractive synthesis The primary colors are cyan (primary blue), magenta (red primary) and yellow : CMY (Cyan, Magenta, Yellow) .
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We can not quote The primary colors for additive and subtractive synthesis without having to explain a little more and in a simple way both principles.
additive synthesis
The additive synthesis is the fact just to use light more sources of different colors to get a new one. To better understand this principle, just imagine projectors, each emitting a color . The new color combination is carried at the crossing lights This principle is used for everything related to the light.. Television, projectors, video projectors, LCD and LED ... The absence of color for synthesis additive is black.
subtractive synthesis
This principle known for its use in paint or printers wich not use light, unlike the additive synthesis, for out The colors but the color is represented by the material. In this case we use the brightness of the medium used for The highlight colors. The principle to get nouvelThe colors is to subtract colors starting PRIMARY COLORS who are cyan (primary blue), magenta (primary red) and yellow. This principle is also known to deliver better results in the areas mentioned above. The lack of colors for subtractive synthesis is white. In printing, it does not use the black obtained by mixing PRIMARY COLORS, it is not a perfect black which explains the use of a separate black cartridge.
The additive synthesis is the fact just to use light more sources of different colors to get a new one. 
This principle known for its use in paint or printers wich not use light, unlike the additive synthesis, for out The colors but the color is represented by the material. In this case we use the brightness of the medium used for The highlight colors.