Additive color produces a color stimulus by superposing light from different sources. For mutually incoherent sources, their spectral intensities add, and the combined optical spectrum drives the observer's receptors. Red and green display primaries can therefore give a yellow appearance without emitting only yellow-wavelength light.
Project two colored beams onto the same diffuse screen and compare their overlap with the separately illuminated regions. Holding the surroundings and adaptation fixed makes the comparison interpretable. The attainable colors depend on the primary optical spectra and their allowed powers, which define a device gamut; merely naming three primaries does not specify every stimulus they can produce.
New to topics? Read the docs here!