At Spectrum Fantastic Art Live here in Kansas City, we had a fun time talking about portable sketching setups.
Then I did a quick demo of a quick paint-sketch technique, thanks to two volunteers who stood looking at their cellphones.
We set the timer for 7 minutes, and I limited my gouache palette to yellow ochre, perylene maroon, and ultramarine blue, plus white. I quickly sketched in the figure with a watercolor pencil. Then, using a synthetic round brush, I painted the silhouette as directly as possible.
Seven minutes were up in no time at all, but that's pretty much all the time you'd get in a real-world situation.