An interesting theory - maybe correct. Another possibility, thinking of stillp's comment, is that the two-colour stripes are probably more immediately identifiable (than would be any single colour) by those with a range of colour vision defects.I can only assume part of Europe was using yellow as earth and part using green Which is why we ended up with 2 colours for earth?
Seeing how the world has evolved, if the system were being invented today, we'd probably end up having distinct 'stripe patterns' for each conductor (so that there was no reliance on colour vision at all), and maybe also Braille markings (for those who had no vision at all, but nevertheless were engaging in electrical wiring ).
Kind Regards, John