Decorate cookies. Prepare royal icing following recipe directions. Tint portions black, light gray (use black color), sky blue and rose/sky blue combination (for rose). Reserve a small portion of light gray icing. Thin remaining icing following recipe directions.
Use tip 2 and full-strength light gray icing to outline cookies and pipe diagonal lines, 3/4 in. apart, in both directions. This will create diamond shapes. Let set 1 to 2 hours.
Use thinned light gray, black and blue or rose icings in cut parchment bags to flow in diamond areas. Use a toothpick to pull icing into corners if needed. Let dry overnight.
Use tip 1 and full-strength light gray icing to pipe diagonal dotted lines over diamonds. Use icing to attach Monster High icing decoration to each cookie.