Next I painted in the dark triangle that would be the water the zebra was standing in and drinking from. At such as initial stage in a painting's development, it's important to be bold, not timid; the dark part may seem too much or exaggerated right now, but there's still a lot of painting that'll happen on top of it which will calm it down.
Then I suddenly worried that maybe zebras didn't do this and only ever drank from the edge of a water source to avoid crocodiles. So I surfed around a bit researching zebras' drinking habits and found that they do on occasion wade into deeper water. Just as well, or I'd have had to change the entire composition. (Note to self: Check this kind of detail before I start painting next time!)