Weasleys Wizarding Wheezes, at No. 93 Diagon Alley.
I like how this one turned out. I used three basic colors to paint this, and I had a ton of fun. (Colors: French ultramarine, raw sienna, and burnt sienna, plus cobalt for some shadows and cadmium red for the lettering on the door.)
Watercolor, no ref. I just always imagined WWW having a blue door.