The Pumpkin is treated as a thatch structure and can be damaged by anything except non-explosive ranged attacks.
Painting and Color Regions
Unlike any other object, drawing on the Pumpkin results in a "carved" look.
In addition to its preset paint regions, you can doodle on the Pumpkin by "attacking" it with a Paintbrush, which will open the "Apply Paint" Menu. Then, click and drag your cursor over the 3D model (like a paintbrush) to draw anything you want. See Painting for more information on doodling.