I do the majority of my work in Paint, and hop into Photoshop to do shadows.
So up until that black line I was drawing exclusively in Paint. Once the shadows were done in PS I went back to Paint and added little touches like window highlights, rooftop antennae, etc. If you compare the drawing before the black line to the final one, you can see how the shadow gradient makes the building seem more realistic and a little less 2D.
Here's a peak of what I do in PS, from the top down it shows the grey wall's shadow gradient and a dark blue gradient over the windows:
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