PyNGL equivalent to gsnFillColors with transparency

[senesis]

Using Ncl, you can use gsnXYFillColors together with colors defined by a quadruplet (RGB+transparency) for creating graphs where an enveloppe between two curves can be lightly shaded, and superimposed on another enveloppe.

This is e.g. useful when showing time evolution of a climate variable, and its confidence interval, under two forcing scenarios

I have not been able to figure out from PyNGL doc how to do that : If ressource nglXYFillColors allow to fill the area between two curves, it only allows for named colors or color indices, and not color quadruplet such as with gsnXYFillColors; and PyNGL explanations about named colors and color indices make no mention of opacity tuning

So I assume this is a PyNGL shortcoming (vs NCL). Insnt'it ? Any hope for a fix, sooner or later ?