When you start from a black screen (eg. a monitor) your primary colors that can form all others are literally red, green and blue. Those are the subpixels that every monitor must have as a minimum. These colors will add to white and mixtures of them can make every color humans can perceive (provided your rgb are at the maximum extremes of each color, otherwise your color triangle is slightly smaller than what rgb can actually do). This is called additive color mixing. Start from black and add color.

When you mix paints your are subtracting from white. The primary colors are 45degrees around the color wheel, namely cyan, magenta, yellow. From those you can make alm other colors by subtracting various amounts of the above three colors from white.

Btw do you know what color model matches your eyes the most? It’s additive color mixing. Your eyes literally start by seeing black and as your eyes let in light the cones and rods that are focused around detecting red, green and blue are triggered in various amounts and you see color that way. You don't see color the same way color is formed from mixing paints. You don’t see cyan, magenta, yellow at all. You see in RGB.

So your painting example is really flawed. Subtractive color models are inly useful for paints.