Colour each layer of the item via the colour button in the menu. Modify the layers (or delete them and start from scratch) using the Points Tool, and learn about the other tools by hovering over them. Click on each of the layers using the Select Tool, and look at the menu to determine the type of the layer.
Note: colliders are always convex polygons (due to physics engine things), but graphics can be any shape and can contain holes. Colliders should be used as a rough hitbox and graphics layers should contain all the detail. When creating a new item, two layers are automatically created: one graphical, and one invisible collider. Each layer consists of points and defines a single shape. MaD Lab at its core is a polygon editor. The best way to learn is to try out the editor, it's relatively simple.
