In gnome-panel/Adwaita.css "theme_unfocused_fg_color" is used as color. So "correct" color is that one you see with in-process / built-in applets.That color is used since this commit:
https://git.gnome.org/browse/gnome-panel/commit/gnome-panel/Adwaita.css?id=7c1607feed985a5fd8d7bff46605ef10bb2a9a1dIf you have time then you could try experiment by using other color "names" available here:
https://git.gnome.org/browse/gtk+/tree/gtk/theme/Adwaita/_colors-public.scss