Now a little hint:
Such old buildings use to get a irregular dark stroke around their bottom caused by soil moisture and other things.
To be sure, print a photo of the building and touch the bottom edges with the tip of your fingers dirty of dry pastel black,
but varying the height of the dark stroke from a couple of millimeters to one or one and a half centimeters.
Then touch here and there some spots from the bottom line up the same way but with moss green.
Also the places where feet and hands touch the windows and doors get a bit dirty...
Of course, the more subtle you do it the more convincing.
But don't get angry if you see what you've done on the printed image is exagerated.
Most experiments start with exagerations (as when God created humankind, poor fellow )
____________________ Extract from "THE FOUR STEPS METHOD" by Ching Pang Tsè:
1) Calm down.
2) Calm down.
3) Calm down.
4) First calm down.