It is the difference between additive mixing and subtractive mixing. When you mix colors on a screen with RGB, you add light. When you mix pigments on a physical medium, you subtract the amount of light reflected (because each paint absorbs most light except the colors it reflects, which are what you see).

As a side note, when mixing in the subtractive color system, your primary colors are cyan, magenta, and yellow. That’s why a printer takes CMYK, for cyan, magenta, yellow, and black. In case you were wondering, ‘K’ here is black.

Great explanation. Thank you.

Can you also tell me how a computer monitor makes Yellow when it only has RGB pixels?

On a spectrum of visible light, yellow has a wavelength perfectly between red and green. Therefore, combining red and green, the average wavelength is the same as the wavelength of yellow.

For reference:

One curious thing if you understand this is to think on purple. Purple is blue+red, but like you pointed out 2 colors should give you the average wavelength, which in the case of blue+,red should be green. So why the hell do we see purple as something different? Well, that’s because humans have 3 sensors for colors, roughly corresponding to Red, Green and Blue, triggering both Blue and Red without triggering green at the same time gets interpreted differently than green, even though it shouldn’t. Which means that purple is not a color, but rather a mind trick your brain plays on you.