All light is comprised of various wavelengths which, when divided, provide a spectrum of color.
The presence of all light waves simultaneously produces white light. The additive primaries are red, blue, and green . As these colors mix, they produce all colors in the visible spectrum. This is how we are more used to thinking about color, though the science of how we see color is why we call it subtractive. Light waves hit an object, whose molecular structure causes it to absorb some light wavelengths and reflect others.