The main reason RGB works so well without yellow, is that the human eye has red, green and blue cone cells, and the brain sees yellow when some specific ratio of response from the green cones and red cones happens. A color CRT produces red and green light in the same ratio, tricking our brain to think that there is yellow light present. But a spectrascope looking at the CRT would only show a spike at green light frequency and another spike at red light frequency, and nothing at the yellow light frequency.