To temper your point just a bit, even though yellow light and red+green light look the same when viewed directly, you can still tell the difference. For instan

To temper your point just a bit, even though yellow light and red+green light look the same when viewed directly, you can still tell the difference. For instance, in the way the light reflects off different materials, or by chromatic aberration with corrective lenses. This is why R+G+B LED lighting is not good enough for illumination and why some better bulbs instead use UV LED + phosphors.