Thank you for all the encouragement.
I gave it another try to weather the old brick structures. I think those old buildings needed the weathering most to represent old structures in modern times (e.g. 1980). Photos of the west Bottoms in Kansas City MO are used for reference. I wanted to avoid irreversible mistakes and used water colors as they are used with small kids in the kindergarten and at elementary school. The colors are mostly used like a wash and the water had a lot of detergent to let it flow well.
This is a closeup at my desk with the used colors in the foreground. It is interesting how the camera emphasized the white wash, honored the black area and barely notices the brown painted spots.
And a photo of the same building integrated into the scenery with a normal viewing distance.
The big advantage is the water color. The building may join me under the shower and we are both fresh and clean again
The East industry is not dominated by the mortal wash and saltpeter emission of the bricks but pure dirt. I tried that with black only. the paint is applied at the very top of the wall and runs down guided by a very wet brush only.
That may be either light
or more robust
In both cases is the water paint mixture mainly searching it's own way down the wall (there is a lot of detergent in the water!). I controlled it only by adding water where necessary.
There is another surprising effect. The black paint water mix drives the lighter old mortal wash out of the joints and lets it set on the face of the bricks. That results in a lighter wall with darker joints with an overall reduced brown but more gray shade. It has some similarities with a light gray dry brush.
It is less visible from the distance
and again if you don't like it... take a shower with your buildings