After the initial sketch, I stopped for several hours. I was intimidated by the white space. Usually, if I am starting a fresh canvas, I start not with a sketch but by putting down an underpainting. Mine is usually some mix of paint colors that are dark. I tend to use paint colors that are less opaque, like ultramarine blue, alizarin crimson and Indian yellow, but really it's also just as often whatever I have the most of that is left over on the palette from my last painting. I apply it as fast as I can with the largest brush that I have and usually rub it with a paper towel so that it's uneven and very thin in places, so the white of the canvas shows through. Basically, I mess up the canvas deliberately, so that I can then be fearless about painting on it.
With this painting, I have titanium white, Naples yellow, azo yellow, transparent yellow iron oxide, naphthol red, anthraquinone red, alizarin crimson, ultramarine blue, manganese blue hue and olive green on my palette. I mixed the paint with a palette knife to get some more neutral colors, then used my three-inch hardware store brush first, then my two-inch hardware store brush, then an approximately one-inch flat hog bristle brush to apply this paint. (I changed brushes mostly so that I could paint quickly, without wiping off the brush in between.) I also used my dark oil bar to paint lines.