Set aside two cups of white icing. Color the remaining icing with brown gel food coloring, adding a little at a time until the color is as desired. To use less brown, add the tiniest bit of black gel coloring – literally a speck will be enough.
Thin icing to outlining consistency. Fit piping bag with #2 tip. Pipe the poop outline, including the swirl. Allow to set completely.
Add a spoonful or two of white icing to the brown to slightly lighten the color. Use water to thin to flooding consistency. Fill a piping bag fitted with a #2 tip to flood the outlined poop cookies. Allow to set completely.
Once icing is set, decorate with candy eyeballs. Use a dab of royal icing to attach them. Pipe a mouth using white icing thinned to outlining consistency.