in the RGB space for subtrattive theory of light , the complemetary of red color is the green color

According to color filter concept, after absorption of specific wavelength, the rest are reflected. This way, when red(near 600nm to 700nm) is absorbed, the rest should be reflected. And the rest combines as cyan, not green. Still I need to be clear at molecular level.

in 8-bit channel RGB which is additive space by the way, red is 255//0//0, its complementary is 0//255//255 so Cyan

in % CMYK which is subtractive space, red is 0//100//100//x (with x describing the darkness of the red), its complementary is 100//0//0//100-x so from Cyan to Teal

I cannot understand why green can be the complement of red

Complementary colors are pairs of colors which, when combined, cancel each other out. This means that when combined, they produce black, or if colored light (rather than pigment) is used, they produce white

good details can be found:

http://en.wikipedia.org/wiki/Complementary_colors

In RGB, and by definition, the complementary color of red is cyan (which is made of blue and green).

There is a good reading resource at http://www.physicsclassroom.com/class/light/Lesson-2/Color-Addition

http://www.physicsclassroom.com/class/light/Lesson-2/Color-Addition

Color, as an objectivated human sensation is a rather involved concept - the Wikipedia article cited by Samuel gives an impression. So it might be worth-while to single out a fact which refers to a simple setting. Let us consider only monochromatic light: The for any wavelength of red light there is a certain wavelength of green light which gives white light by superposition. Since what is white to one visual observer may be yellowish or bluisch to an other observer, the wavelength of that green may be not exactly the same for a large ensemble of observers. For practical purposes there are agreed objective procedures which replace the subjective judgements of human observers.

Thanks Ulrich for your contribution.

From the visual perception Point of view I can only agree to your answer.

But if we agree to define (oppsed to perceive) as

RGB 255//0//0, then its complement is 0//255//255 which is Cyan, and the same happens in CMYK.

Would you agree then to the Statement that for the Computer defined red, its complement is cyan?

Even for pigments, I cannot understand Wikipedia Statement that red-green complements

(off-the-record note, since I do not trust all that is written in Wikipedia, I could here blame on this Wikipedia article, just because I do not understand it, but this would not be fair either)

Even in the Boutet circle they present, you see in the left Panel a 7 Colors ring, so no opposite concept can be applied here, and Rouge de feu is placed opposed to... the midline between blue and green (Cyan!)

in the right Panel, there is a 12 colours ring, and there Rouge de feu is placed opposed to Verd de mer (Cyan!)

So it seems to me, that Red-Cyan is the real complementary pair.

Dear Samuel,

all you write is right for some concept of Cyan. Actually the green which mixes to white with a yellowish red (Orange, 'Rouge de feu') is a bit blueish and could well be called Cyan. Note however that we here consider monochrome light and that therefore the hue wich is between Green and Blue is not a mixture of Green and Blue: it is not a mixture at all - it is a pure color. One of my reference books (R.W.G. Hunt: Measuring colour) calls this spectral blue-green not Cyan but Turkoise. For the restricted colorspace of computers with the arithmetic definition of adding colors (actually dependent on 'gamma') the statement Red+Cyan =  White (or Gray) is obviously right.

Thanks gain Ulrich, I always enjoy your contributions.

Absolutely right in your appreciations, in particular making clear that "in between" in Terms of monochromatic light is not "mixing", but I hope my words at least let Interpret this in between as a progressive "in between".

Also perceptionwise, and due to the Absence of "exact" definitions (though all definitions are exact) here, it is difficult give a Name to each "colour", and impossible to give a name to every possible wavelength...

But in Spain "verd de mer" (verde mar, sea/ocean green) is sometimes/often referred to as azul turquesa (turkoise), and this is as you stated also quite similar to Cyan (but in my opinion more different to what I personally would call "pure green")

However, I think we could not and cannot give a definitive answer to the original question.