When you mix paint, you mix pigments that each absorb certain colors, so they won’t reflect it. You’re subtracting colors to the ambient white light that reflects a smaller and smaller portion of the spectrum on the painted surface, and if you mix enough colors you substract most of it and get a dark brown or even black. When you encode rgb values for a screen, you tell pixels to add colors to the light your screen is generating, and if you add enough you get a white light.