Colour computer screens use RGB - colour printers use CMYK. (K is ‘black’, which is not a primary or secondary colour, so we’ll ignore it for now.)

They nest into eachother, with RGB being primary colours and CMY being secondary: R-y-G-c-B-m-R. Any kid that’s used fingerprints or water colours knows how to combine primary colours to get secondary colours.

You know if you want red, then you do 100% red. You know if you want yellow, you do 50% red, 50% green. If you want orange, which is a more reddish yellow, you do 75% red, 25% green. Brown might be a little more complicated - but still, what is brown? Darker orange? Maybe 25% red, 15% green? Try it and see!

(Also for black and white - again, any kid with a prism and a flashlight (or sunbeam) knows that white light is all colours together - red 100%, green 100%, blue 100%. The opposite of white is black, which is no light at all, which is just 0% across the board.)

No math adeptety or cube visualization necessary, this is all elementary school children level stuff. You already know most of it.