If you look at all the level 6 buildings (max upgrade) - they all have a green/darkgreen roof or some green shades to them.

This helps me glace over my base and see which ones of my buildings are fully upgraded.

The Improved Watermill is the only one that doesn't have a green roof. I believe this should be adjusted to have green roof or some sort of green on the building. Maybe the water tower on changing the gold. Currently it looks exactly like the level 5 version.