I always remember learning that the three primary colors are Red, Yellow, and Blue. Many people rather insist that they are Red, Green, and Blue.
When you take paints or markers or something else along those lines, combing RYB will give you all the colors, whereas RGB will not. Furthermore, electronic displays and lights use RGB as their primary colors, not RYB.
So what exactly is the “true” set of primary colors. Or are there just two sets that function differently?
In: Technology
Colors are wavelengths of light. The human eye can see wavelengths from 380 nm (violet) to 700 nm (red). Other wavelengths aren’t visible to humans, but you can capture them with special cameras.
To distinguish between colors within that range of visible light, the human eye has cones that detect specific wavelengths more strongly than others. The cones are centered around the wavelengths of red, green, and blue.
So in a sense, the eye can only see red, green, and blue. When we see yellow light, what’s actually happening is that both the red and green cones in our eyes are equally stimulated.
If you mix red light and green light together, it looks the same as “actual yellow” to our eyes. It’s not ACTUALLY the same physically, but the human eye can’t tell it apart.
So that’s how TV screens (and computer screens and phone screens) work. They use a combination of red, green, and blue light to simulate every color the eye can see. They do NOT simulate every color in nature, but because our eyes only sense red, green, and blue, we can’t tell the difference.
So in that sense, the primary colors of light are red, green, and blue.
Paint is different. When you shine a light on colored paint, it’s absorbing some colors and reflecting others. When you mix two colors of paint, you get a mix of their absorbing and reflecting properties. It doesn’t work the same as light at all.
The proper red, yellow, and blue paints are good primary colors. You can make any color of the rainbow with just those three.
With ink, it works better to use cyan, magenta, and yellow as primary colors.
RYB are the real primary colors.
RGB are the primary colors for video technologies because the screens emit light, which is countered with that extra blue.
Printers use Cyan Magenta and Yellow (CMY) because they layer the colors to make darker versions since they cannot be made to be lighter.
It’s a different function. If you’re talking about additive colours, as are used on your computer display, then you use RGB–red and green together will make yellow, etc. For printing, the colours are subtractive (they’re absorbing some part of the incoming light), and the base colours there are actually cyan, magenta and yellow, not blue, red and yellow.
There is no RYB: printers use CMYK (cyan, magenta, yellow, black). Kids get taught that it’s red and blue because those are familiar colours but the technical names as magenta and cyan. CMYK is the system for use with paints and inks, i.e., for subtractive colour mixing. You start off with a white canvas and each layer of paint subtracts a colour:
* cyan + magenta = blue
* cyan + yellow = green
* magenta + yellow = red
* cyan + magenta + yellow = black (but not a very good black, so we use black ink)
If you’re starting off with a black screen and using light to add colours, then you use RGB (red, green, blue). This is additive colour mixing as used on computer monitors and TVs.
* red + green = yellow
* red + blue = magenta
* green + blue = cyan
* red + green + blue = white
So there are two completely different (in a sense opposite) sets of colour primaries: additive and subtractive. The one you use depends on how you’re creating colour.
Primary colors are simply any set of colors that can reproduce all other colors by mixing them. The way of mixing defines what colors are primary colors.
In paint the way of mixing is subtractive as each pigment of paint – absorbs – “all but one color” so a red pigment absorbs all light quite good except for red light which is reflected back to you, thus you see the paint as red. So when mixing paint together more colors are absorbed, thus more colors are subtracted, from the color pool.
Due to this behaviour the primary colors of subtractive color mixing of paint in art and design are Red, Yellow and Blue. (This is not the whole spectrum though)
However everything that – emits – light mixes by – adding – colors to the pool. So a red light emits “only in the red” and a blue light “only in the blue”. Thus wenn mixing them together you receive blue and red data. So this way of mixing is – additive – and here the primary colors are Red, Green and Blue.
So to say “those are the primary colors” without mentioning the way of mixing is simply wrong. The definition literally states “any of a group of colours from which all other colours can be obtained by mixing”. Thus there are no “real” or “unreal” primary colors. It simply depends on the way these colors are mixed and if all other colors can be obtained.
So in the end what colors are considered primary depends on the so called “color model”. The most common ones are the paint (subtractive), light (additive) and ink (subtractive) models.
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Primary colours is basically just a set of colours that produce some *gamut* of colours by mixing the primary colours. This means that basically any set of colours can function as primary colours for the gamut that these colours may create. You can technically even have only two or one primary colour for a gamut, although that would be pretty useless (in the case of only one primary colour, the gamut would consist of only that colour).
One important fact to note is that it is practically *and* theoretically impossible to create *all* colours from a finite set of primary colours. So no, there are no 3 (or any finite number of) colours that can create all other colours.
Any colour gamut is fundamentally arbitrary, but we of course try to create gamuts that cover as much of all visible colours as possible using the technology we have. RGB is often used in screens becasue it covers a large amount of the visible colours and we have the technology to create red, green and blue pixels using e.g. liquid crystals (LCD) or LEDs.
Take a look at this review of an LCD-panel over at Rtings: [https://www.rtings.com/monitor/reviews/lg/27gn800-b](https://www.rtings.com/monitor/reviews/lg/27gn800-b). Here, they look at at how well the monitor covers at least 4 different colour gamuts, all of which are a type of RGB-gamut (since the panel has red, green and blue pixels): SDR gamuts “sRGB” and “Adobe RGB”, and HDR gamuts “DCI P3” and “Rec. 2020”. Of these gamuts, Adobe RGB is wider than sRGB, i.e. it covers more colours. Similarly, Rec. 2020 is wider than DCI P3.
Regarding human vision, we have three colour photoreceptors. Contrary to popular belief, each receptor can actually “see” more than one colour. Each receptor responds differently to different wavelengths, see this picture from wikipedia: [https://en.wikipedia.org/wiki/Primary_color#/media/File:Normalized_Cone_Sensitivities.png](https://en.wikipedia.org/wiki/Primary_color#/media/File:Normalized_Cone_Sensitivities.png)
Take a look at the L-receptor. It responds most strongly to yellow, but it also responds “sees” red and green, and even blue although very weakly. Depending on the responses of the three receptors, our brain constructs an image in our minds. But again, there are no “true” primary colours. Animals may have different, or even more, photoreceptors, which for example allows certain birds to see ultra-violet, i.e. colours that humans *cannot see* at all.
As others already have explained, creating colours from primaries also function differently depending on whether you use additive mixing (you produce light using some sort of lamp, e.g. a LED or LCD) or subtractive mixing (you remove light by adding e.g. ink/paint). Note that additive mixing is in a sense independent, i.e. the lamps you use shine with the colours that they shine, period. In subtractive mixing, you have some light source (e.g. the sun) that shine on your painting or whatever. The light from the sun is a mix of colours, and then you add paint to *remove* some of the colours from the light so to speak. This means that the colours you get depend on the colours existing in the light from which you remove colours, and also their relative intensities. If you swap the sun for a blue-ish LED-lamp, the colours of the painting will look different even if you have the same paint on it. Basically, a red painting only looks red as long as the light source shining at it actually contains the colour red (or some combination that the human brain would percieve as red).
Here is a link to the rather good wikipedia page on primary colours, which I base a large part of my answer on: [https://en.wikipedia.org/wiki/Primary_color](https://en.wikipedia.org/wiki/Primary_color)
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