The Yellow Hues of reflecting surfaces in nature and everyday life are due to the fact that these surfaces absorb Blue light from the White light falling on them. Red and Green light are the only Hues reflected from these surfaces.  Then the Red sensitive cones, and the Green sensitive cones in our eyes each recieve equall stimuli and we percieve the sensation of Yellow, without actually seeing the yellow of the spectrum.

When looking directly at the rainbow, the Yellow of the spectrum must have the right amount of energy to affect both the Red and Green sensitive cones in our eyes equally. Here, the rainbow is a dispersal of white light, and the Yellow is not due to the subtraction of blue light from white light,- nor is it due to the equal presence of Red and Green light which both appear elesewhere in the same rainbow.