For 3D Printing I needed the color of the original cases of the XE and ST Series. So I bought a color checking device RAL RAL Color Reader and a RAL Farbkarte.
I checked several Atari XL Computer in different state of yellowing.
The best looking NOS Atari on the left side has the following colors.
RAL 1013 Perlweiß is best fitting for good looking Atari XL Computer not yellowed
For more yellowed units
RAL 1014 Elfenbein fitts for yellowed units like the right one.
First try RAL 1013
https://www.3djake.de/extrudr/green-tec-pro-natur