This is because even some real princeps we see on the internet are not really blue,more bit of like a green palm with only white backs. and in some cases, even the undersides ares just blue or gray, not so sharply white.
Actually, T.princeps is a blue palm with white powder to the leaf undersides ,uppersides and petioles. My several princeps grown in the ground and pot out of seeds from the stone gate demonstrate how blue white they could be at both sides.
Though a blue T.princeps with white powder at upperside could be far more interesting and preferable than a green one with only white underside which is not rare to see , it is still mysterious and difficult to find out what the main reasons are behind the variations.
Maybe someone could contribute his growing experience as to shed light on this.



