It will get just as “dark” if it is cloudy or sunny. It will just not be as a “dramatic” a change.
How dark - perhaps similar to Civil Twilight, closer to the of the End Evening Civil Twilight spectrum?
A lot depends on the umbral width.
I’ve never seen ‘the world’ quite the color it was during the 2017 eclipse. It was surreal. The whole ‘atmosphere’ felt very strange.