Decorate cookies.
Pumpkin Cookies: Using either thinned white or black icing, pipe center section of pumpkin and two outer sections. Let set, about 10 minutes. Fill in remaining sections with other color. Let set, about 10 minutes. Using light purple, light blue or thinned light pink icing, pipe stem.
Cauldron Cookies: Using thinned white icing, pipe highlight on side of cauldron. Using thinned pink icing, pipe small oval on top of cauldron, leaving space for black rim. Let set, about 10 minutes. Using thinned black icing, pipe rim around pink oval. Let set, about 10 minutes. Using thinned black icing, fill in rest of cauldron. Let set, about 30 minutes. Using full-strength pink icing, pipe bubbles.
Bat Cookies: Using thinned pink icing, pipe body. Let set, about 10 minutes. Using thinned pink icing, pipe wings. Let set, about 30 minutes. Using full-strength pink icing, pipe lines on wings. Using full-strength black icing, pipe eyes and mouth. Using full-strength white icing. Pipe fangs.
Web Cookies: Using light purple icing, fill in web. Using thinned black icing, immediately pipe web lines, starting with straight horizontal and vertical lines, then diagonal lines. Connect lines by piping two curved lines in between, forming web shape.
Tombstone Cookies: Using thinned blue icing, fill in tombstone. Let set, about 30 minutes. Switch full-strength white icing to tip 4. Pipe message.
Let all cookies set 24 hours.