Like others have said, it's about additive vs subtractive color
And to start off with, probably everything you know about color is probably over simplified, or even outright wrong. Light and color and how your brain interprets that information is pretty complex stuff. Even this explanation is gonna be glossing over things.
Starting from the basics, white light contains all of the colors of the rainbow.
Your eyes, however, are mostly only sensitive to red, green, and blue light, most people only have receptors in their eyes (cones) for those 3 colors. They do pick up a little bit from the surrounding parts of the spectrum but not much, and your brain sort of fills in the gaps from there. If your red cones and green cones are both getting stimulated by light, your brain will interpret that as yellow or orange depending on just how much each is picking up.
So your monitor is starting with no light across all 3 colors (black)
And then adding light to get the desired colors.
But if you're drawing or painting, ou're starting with a white canvas, not a black monitor, so how do we go about getting the colors we want!
Well we're going to put paint or ink on the canvas to absorb the colors we don't want.
Back in elementary school art class you probably learned about complementary or opposite colors. Unfortunately the colors you learned were kind of wrong. Close enough for kids mixing finger paints, but not exactly.
The opposite of red isn't green it's cyan.
The opposite of green isn't red, its magenta
But the opposite of blue is in fact yellow, so one out of three is something I guess.
What does that actually mean though? Well yellow ink absorbs basically all of the blue light while still reflecting red and green.
Cyan absorbs all the red light, while still reflecting blue and green
And magenta absorbs all the green light while still reflecting red and blue
So by mixing and matching those 3 colors, you can dial things down from 100% white light to a mix of red green and blue that your brain can interpret as other colors.
In theory mixing a bunch of those 3 colors together, you can eventually get down to black, in practice your pigments aren't perfect, and even if they were it would get expensive to use that much of those 3 pigments which is why most color printers are CMYK, with "K" standing for black for reasons I've never bothered to look up and I'm not gonna start now.
So your monitoring is adding light from 0 up to make the color you need. It's "additive."
And paint is dialing things down from 100 to the desired color. It's "subtractive."
Hopefully that all makes sense, color is weird.