We have other plans for colored folders - namely the new grid design. With that, we no longer need icons and can make folders gray instead.
That didn’t make it to 47 tho, so we have a fallback: the selection color in nautilus and gtk file chooser is now grey - you don’t have that change yet, probably should just update.
Hopefully for 48.
Colored folders in icon theme are not happening - it’s a lot of work for not a lot of gain when we want to stop using full color icons there anyway. Besides, there’s a precedent in macOS keeping the folders blue, in Windows keeping them beige and so on. Like, blue folders aren’t the only colorful thing in that view atm - we have red videos, orange images, green audio etc.
Ofc if someone wants to make an icon theme for colored folders and/or a shell extension to switch them in sync with the system - they are welcome to. But yeah, by default we’ll stop using icons in nautilus next cycle.