Did you change your wallpaper? The folder colours are based on the General colour of your wallpaper.
Here's one of my folders, see how it's colour is related to the background colour of the wallpaper.
On the other hand, if your folders are a dark grey colour, you may have accidentally activated the reduce transparency feature as in this screenshot.
To check, go to Settings>General>Accessability>Reduce Transparency. If the switch is green, reduce transparency has been activated. Turn it off to restore it to what it was.