If you matched the yellow light of one prism to that of the green light of another prism, say through a thin glass pane, what would be the resulting color?
Irrespective of where the light comes from, your eye judges the colour of light by the way it stimulates the three types of cone cell. The cone cells have broad and overlapping spectral responses:
(picture from the Wikipedia article I've linked) but to a reasonable approximation you can think of the three types of cone cell as responding to red, green and blue light. This is why the LCD screen on your phone can reproduce a wide range of colours using just red, green and blue pixels.
In the experiment you describe the yellow light is not a mixture of red and green, it's light of a wavelength of around 580 nm. However it stimulates your cone cells in the same way as a mixture of red and green light does, so a mixture of red and green is seen by your eye as yellow.
So if you mix green and yellow light then as far as your eye is concerned you are mixing two parts green light with one part red light. The corresponding RGB value would be (128, 255, 0). If I use my graphics program to produce this colour I get:
So it's just yellowy-green.