I understand that the 320 recoil springs for 9mm and .40 are not the same. The springs have colored paint on one end - which color is for which cartridge ? ( I managed to set some down and may have mixed them up )

The color codes go back into the Classic line era. There have been some variations based on models, but most commonly and for my full size P320s:
Orange: 9mm Parabellum
Green: 357 SIG and 40 S&W

Thank you for the quick response ! I picked up a .40 cal exchange kit for a 320 f (originally in 9mm ) and managed to set the 9 and .40 springs down next to one another. And as others have stated the factory barrel diameters are not the same.

M17 springs at the Sig Store have a blue mark. Not sure how they are different than the red springs that came with my two M17 Bravos (ca. 2018/2019 black finish). Only thing I can see is that the guide rod is parkerized instead of blued. Spring rate might be slightly different for NATO ammo, but it's not easy to tell any difference.

Thank you for the information.