In RGB, (127, 127, 0) is not a "pure" yellow, it is just the color you get when you mix red and green primaries. The human eye is actually rather poor at spectrophotometry, which is why colorimetry and spectrophotometry are distinct subjects.
However, when you say "pure white light"... there is no such thing! White light is a combination of different wavelengths of light. You can indeed make light that looks white (and therefore is white) by combining monochromatic yellow and monochromatic blue light sources. Amazing! Even though it will look white, objects illuminated by this unnatural light source will have unexpected colors. This is quantified by the CRI of a light source. Incandescent lights are defined to have a CRI of 100, but this is just a convention: the blackbody spectrum is quite common. If you grew up under fluorescent lamps, colors under the light of day might seem quite unusual.
