Shape tweens work better when the shapes use thick lines. Thin lines don't morph as smoothly.
When you use a single color your morph will change smoothly and create interesting patterns
If you use multiple colors in both the beginning and ending keyframes of your morph they will blend together as they change
You can also use the timeline to make things appear and disappear on the stage of your movie
You can see the edges of one drawing gather together to change into the next