Subway Riders, in progress

I've been working on a 30" x 40" painting of a subway scene: two riders and a driver, with a third passenger visible as a reflection in a window.

The final drawing of the composition:

Subway Riders, drawing #5

30" x 40"    19 -21 January 2014      pencil on paper w/ pastel tone on reverse
Private Collection, Massachusetts

I use the final drawings to transfer the image to the canvas, so putting pastel on the back is purely utilitarian.  I lay the drawing over the canvas, trace the lines, and the pastel acts like carbon paper. When finished and the drawing is removed, I'll lightly pencil over the pastel lines left behind. This allows me to keep the amount of pencil work on the actual canvas to a minimum.

Here is how the painting looked two weeks ago:

Subway Riders, in progress on 26 March 2014 

And a week ago:

Subway Riders, in progress on 3 April 2014

I try to keep unfinished areas clean and white, and never underpaint or block in colors beforehand. I want my colors to be as luminous as possible - as if the light source was from within and behind the image - and I think working this way contributes to that end.

The desire to leave unpainted areas of the canvas as white as possible is also one of the reasons I don't like to draw directly on the canvas beyond what is minimally necessary.