It's important to understand that colour sensation is entirely psychophysical. The colours we sense are a product of the construction of our eyes and nervous systems - they don't really exist outside the context of a human observer (or an observer with the same visual architecture).

A spectrometer can easily tell the difference between (light of wavelength 580nm) and (mix of light with wavelengths 480nm and 650nm). There is a physical difference. But human vision cannot tell the difference - our brains sense both these as "yellow". Which one is really yellow? Neither. "Yellow" has no physical reality outside a human brain - it's our label for a sensation that we have.