It’s basically a pep-talk/lecture with cute illustrations.  The basic theme is there’s no wrong way to paint: it starts by insisting any idea is fine, then goes on to describe different methods different painters use.  It assures the reader that it’s normal to feel cross if it doesn’t turn out as you hoped, and insists that the important thing is to have fun.  The bright colors in the illustrations will lead children to pick it up, but many will be disappointed when they read it: it’s a bit preachy. But it does have a good message.