This is how I often begin a painting... with a drawing on the canvas.
I start by laying in some of the darks, this is all done with burnt umber --- loaded brush for the darks --- the lighter passages are scrubbed in with the brush after I wipe most of the paint off with a paper towel.
Here's a closer shot of the drawing, that shows the coral that goes across the bottom of the painting,. There's not really a lot of detail... just trying to establish a sense of scale and a pattern foe the lights and darks. This is usually all I do in the first session, it takes about an hour.