Can anyone know what the color code and finish type would be (factory gray) for a PE 90 / 550?

I have blended KG cote colors and Cerakote colors and gotten close but, as far as I know, there isn't a go-to outside of the factory Swiss paint. I don't know how to spell it properly, so I won't try. I know it's hard to get nowadays. I have heard tell that you more-or-less can't get it at all now, due to EPA stuff. Some guys may still have some though.

Not to mention the fact there's a few different hues on the factory guns. The greener, darker hue, the grey, and I swear some newer guns have a brighter grey.

As KSGM says, it really varies. I have some that are shiny green/grey, a matte gray and new production PE90 is dark grey while others are whitish grey.

Here's a WCA 551 that I had the top half painted after welding of diopter base. You can see the 90's era lower is different than a custom blend to match. Most factory guns don't match on all parts so being a bit off is somewhat normal unless all parts were painted from the same batch. (I have a factory pre-ban 551-2 with dark green gas block and front sight but grey otherwise)

Thank you, I didn't think about how different hues of gray would change from batch to batch or year to year. Looks like my best bet is to take it to a Cerakote refinisher and have them either color match the existing color or have the upper and lower refinished together.

The correct factory color code is: RAL 7022 umbria grey

That's a good tip, on the color code. Given your location, I imagine you have that on good authority. Seems as though it is certainly gettable, albeit not in the unique factory paint type.