I used a scuadra true colors grey card, lightroom... In pure sunlight. No color saturation added, and final contrast compared with the keys in real sight.
OG dyesub with HADapter kit, they nailed the colors of the mods! It is PBT so zero yellowing.
OG Olivetti, they are doubleshots of course, and doesn't look yellow at all, along with the KBD2812 grey case. Pretty good state.
I can't find correspondance with modern GMK colors.
BJ seems the closest but looks too yellow
T9 looks less yellow than L9 but looks too bright.
Good luck with color matching those OG Olivetti!