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
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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