I tried increasing overcast day contrast with Curves adjustment.
I picked a spot on the lighter limestone and brightened by pushing the curve down to brighten.
and a darker area in the brick and pushed the curve up to darken.
Then mask the sky to restore original cloud detail.


forum link: http://forums.dpreview.com/forums/read.asp?forum=1006&message=21440021
original by buckeyephoto: