Why green = blue + yellow, but RGB yellow = green and red?

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Why the color formation in the RGB does not follow the same pattern of nature, and yet it works?

For example:

In RGB, yellow = red and green: #FF0000 + #00FF00 = #FFFF00. But if we take a yellow pencil and another blue pencil (or paints, brushes, etc.), and paint the same area, it turns green (or at least something similar).

The shades are not the same, because it depends on the amount of yellow or blue, to form the green.

That is, the yellow depends on the green, and in another the green depends on the yellow.

NOTE:
I understand that the answers will shift to physics and biology. But I always had this doubt because in addition to working with back-end, I work with front-end, and it occurs to me very often. This is because the RGB system does not "close" much what we learned from the "aunt of Arts" in school. So, it doesn’t get very intuitive when it comes to reaching a color when drawing a page. I’m after why the RGB system is different from the other, and it is chosen to be applied in images and drawings of applications. (If I’m not mistaken, it’s studied in Image Processing).

colors rgb
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    http://riftlabs.com/2010/09/how-rgb-light-works/

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    Light emitted vs Light reflected, I believe...

  • The answer given by @brandizzi already clarifies the question. One case is color subtraction, the other addition.

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    For those who care, here an interesting post (in English) explaining how colors "work" differently: a) as ink; b) as light; c) as electromagnetic waves; d) in our brain.

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    These are not the colors of nature, but the colors of the painters "of the old" and "aunt of the primary". It is not very scientific to choose these colors, the one of CMYK is more precise. Worth a researched in the functioning of the retina of humans, in particular of cones and rods. (recalling that in other living beings the spectrum is different, and that there are some cases where people have different perception of these colors and even more spectrum variation).

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    The question is interesting, but is it on-topic on a programming site? It’s okay that part of the fact that RGB is used in monitors and therefore common when programming, but the answers seem to be about physics, neuroscience and even retinal morphology!

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    @bfavaretto Ah, but just now I was going to ask "Is the blue I see equal to the blue that other people see?" /rofl

  • @Bacco the truth is that the blue you see, I can see it in another completely different way, the point is that that "pattern" you see associates the word blue :)

  • This question is not out of context with regard to programming, it is only limited to the Color Theory which is, for example, one of the points used in character recognition algorithms or even images (see Google Image Search).

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2 answers

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The "pattern of nature" of blue, red and yellow is actually a basis of subtractive primary colors. Subtractive primary colors form other colors through the absorption light. For example, if you mix yellow and blue paint, we have a darker green color, right? This is because the new pigment absorbs both the yellow and blue frequencies - the frequencies that remain and are reflected "cause" the feel of the green color. So the more paint you mix, the darker the pigment will become, because it will absorb more colors.

Computer screens, however, do not reflect light - on the contrary, they emit light. In this case, "mixing the pigments" actually implies emitting light at various frequencies. This means that the more colors are emitted, the lighter the color (because there will be more light). Of course the combination could not be the same as the subtractive colors - in fact, it is quite the opposite: to get the combinations of additive colors, just do the "reverse operation" to the subtative colors. As well says Wikipedia:

The most commonly used additive primary colors are the secondary colors of the most common primary subtractive colors, and vice versa.

Diagrama de sobreposições das cores primárias aditivas amarelo, vermelho e azul (Fonte: Wikimedia Commons)

Overlay diagram of additive primary colors yellow, red and blue (Source: Wikimedia Commons)

As RGB primarily serves to represent color on monitors, it uses additive primary colors.

Now, it is worth noting something important: although they teach her in school, a subtractive color base with red, blue and yellow is bad, because it is not orthogonal. You yourself noticed this by saying that an area painted with yellow and blue "becomes green (or at least something similar)." This base is known as RYB and is taught more for historical and perhaps didactic reasons than for being an efficient basis.

A base subtractive primary colors much better, and that is the most used in practice, are yellow, magenta and cyan (besides black, not to waste ink).

Diagrama de sobreposições das cores primárias subtrativas amarelo, ciano e magenta (Fonte: Wikimedia Commons)

Diagram of subtractive yellow, cyan and magenta primary color overlays (Source: Wikimedia Commons)

This base is known as CMYK (from "Cyan, Magenta, Yellow and Key (black)") and is extremely important in design and printing. If you look at the cartridge of an ink printer, you’ll probably see these four colors. (Black goes along because, although it can be composed in theory with the other colors, it is a much used and pays to be sent apart, in addition to facilitating the darkening of the other shades. There may also be cartridges of other colors, depending on the uses of the printer.)

Repare nas quatro cores no canto inferior direito da imagem. (Fonte: Wikimedia Commons)

Note the four colors in the bottom right corner of the image. (Source: Wikimedia Commons)

This post is a fascinating exploration of this theme (and my main source :P )

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    The interesting thing is that the absorption of colors also explains why dark objects usually get hotter in sunlight (that old black car x white car story). A white object repels colors, already a black object absorbs all spectra, which in a way are also energy, which ends up making the body that absorbed more colors heat up more easily.

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    I liked the answer, but I would emphasize more the fact that the "colors of nature" are actually the colors of the "aunt of the primary". Interesting would be to mention the cones and rods in the retina, that would be easier to understand the reason. Qq way, +1

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    @Bacco I commented at the end on how these colors are not a good base, but I agree that this deserves more emphasis. I tried to make it more clear. The only reason I didn’t get into the rod business was because I’ve gone way beyond my area of expertise (I’m one of those who, if I could, would only be in the command line terminal :P)

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Basically because they are different color systems, additive (RGB) and subtractive (CMYK).

Additive colors

Used in monitors, it is generated by mixing various compliments of light waves. In the additive process black is generated by the absence of color and white is the mixture of all of them.

RGB color model

The RGB color model defines the colors using the components:

  • Red - Red (R)
  • Green - Green (G)
  • Blue - Blue (B)

The RGB component generates the color according to the amount of red, green and blue light, ranging from 0 to 255. When the total value of all colors is added, ie RGB(255,255,255) is generated white color, and when there is no lu, in caseRGB(0,0,0) forms the color black. Sistema de cores RGB

Subtractive colors

Used in prints and paintings. Has as primary colors:

  • Ciano blue (C)
  • Magenta (M)
  • Yellow - Yellow (Y)
  • Black - Black (K)

In the subtractive process colors of white light are absolved, when white light reaches an object part is absolved and part is reflected, unlike the RGB, white is absence of color and black is the mixture of all.

sistema de cores CMYK

The printed materials are reproduced using the CMYK color model. When combining cyan, magenta, yellow and black, so that the value of each component is 100, the result is black. When the value of each component is 0, the result is pure white.

Completion

When we talk about color, we have the color obtained additive (light / RGB) or the color obtained subtractively (pigment / CMYK). Hence the difference in the combination.

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