Basically there are two color models, additive and substractive. RGB is additive - turning on red it triggers the red receptors in the eye. If you turn on all three it triggers all your color receptors and you’ll see white. That is all the visible spectrum, if you look at the Sun (which you shouldn’t) you perceive it as white even if the maximum of the emission is at green.
When mixing paint (subtractive model, like CMYK on printers) if you mix all colors, they absorb different part of the light and don’t reflect anything visible for you, and you’ll see black(ish)