I was able to prevent the blue tint for my Tab Refresh extension while keeping the desired greyscale color to match Safari's native icon color. I added a 2.5px blue stroke to the outside of my vector arrow. Small enough that you don't actually see it, but apparently large enough for Safari's processing to think the icon should be rendered in it's original color. Hope this is helpful for others seeking a solution.

For a group container, you have to supply the full file path, along with the bundle name. For example $ defaults read /Users/<name>/Library/Group\ Containers/<groupID>.group.com.<appname>/Library/Preferences/<groupID>.group.com.<appname>.plist

@Elia314 Did you get this working? Feel free to shoot me an email at "sij" + "itsma" at gmail. I worked through all this and got it working.

I notice that Google Drive uses NSFileProviderCustomAction in some of their helper scripts, and wondering if this is another way to add contextual menus to files/folders in the Finder? Their contextual menus don't seem to be affected like Dropbox's do, which uses FinderSync. I can't find any examples of how to use the NSFileProviderCustomAction approach. Is this a red herring, or is it another way to add contextual menus in the Finder? Any sample code to use to get started?