A Spaniard posted in F/B a pictural form a natural population near Valencia. Among the plants you could see also a specimen that has glaucous leaves both on the upper and lower side. Next to it another individual is green or greener. So imo it has to be some genetic differentiation. Also the progeny from the famous blue individual in Barcelona is becoming bluer and bluer all the time. So it IS GENETICS. Last three pictures here are from my own palm and have been taken very recently. Colors are true!