When I started there was at least some bright sun, by the end it was totally overcast, and no matter how much I try to push it. the grey just gets in. I think what happens is that the bright light makes the mid and dark tones darker, or at least the way your eye sees the darks. When the cloud cover diminishes the brightness, you can see the details in the mid and dark tones essentially making them less dark.
There are a lot of old places like this around here, I'll have to paint more of them!