By reference to the light wheel we can see the result of mixing primary colors
By mixing light we can produce almost any color, including magenta (purple) which does not appear in the visible spectrum. Some of these combinations seem unlikely - it is hard to imagine how you get yellow from red and green. Don’t worry about the why or how though, just accept that white light is a mixture of all three primary colors It should be noted that mixing light behaves differently to mixing colored pigments, as the artist does.
Solid objects appear colored because they reflect some wavelengths and absorb others. A green object for instance will reflect green and absorb blue and red. Objects that reflect no light are black, objects that reflect all wavelengths of light are white.
The additive colors and subtractive colors are complementary colors. Each pair of subtractive colors creates an additive color.