It's gouache! This piece is laying it on thick, with little water, and would cover lines. Obviously some colors cover better than others. You can also water it down and use it more like watercolor. It doesn't really cover the lines this way, but it dulls them. I typically go over them again after the paint is dry. See [link] - I went over the outline and the area around the eyes after the paint dried, but not some of the internal areas.