I hear you!
For my personal collection, I often take the model paint to the hardware store and have them match it as close as possible. It won’t always be 100% but on a large scale its still consistent. We’ve done it so many times now they have some of the Army Painter names in their database (we can just ask for a pot of dungeon grey!).
Matching photos can also be hard when lighting conditions can alter the look of an image so much but it’ll still get you most of the way there.
I think you’ve provided an excellent starting point for many people looking to match the factory schemes!