In 1935, Arthur Guptill demonstrated a sequential method frequently used by architectural illustrators for painting a window.
Guptill first draws the subject in pencil, giving careful attention to the perspective.
The dark interior spaces are painted over the mullions, but not the sash. The dull orange color of the interior gives a feeling of depth and transparency.
If you're not familiar with a ruling pen, it's a tool for drawing a line of constant width, usually guided by a ruler that's raised a bit off the surface. It has two sharpened metal tips that taper together. The spacing of the gap between the tips governs the width of the line.
That gap is controlled by an adjustable wheel on the side. Ink or watercolor, applied by an eyedropper (Edit: or a brush), sits in the gap and flows by capillary action.
Edit: After reading the post, blog reader Glenn Tait got out two of his old Kern ruling pens, ran some tests, and sent the photo, saying: "By rote, after so many years, I filled the pen by loading my brush (using M. Graham watercolors) and pulled it across the open side of the pen to fill it then, after adjusting the tip width, dragged it a bit along my left forefinger to get the flow started. The watercolour works great."
Thanks, Glenn. I'm going to throw one in my sketch kit, too.
Arthur Guptill's classic book about rendering architecture in watercolor is called Color in Sketching and Rendering.