Combine butter, granulated sugar, brown sugar and egg yolk in medium bowl until well blended. Add flour, baking powder and salt; mix until well blended. Tint dough with food coloring to desired shade of green; shape dough into disc. Wrap in plastic wrap and refrigerate about 1 hour.

Preheat oven to 350°F. Line cookie sheets with foil; lightly grease foil. Roll dough on lightly floured surface to 1/4-inch thickness. For turtle shells, cut 6 circles of dough with 4-inch round cookie cutter; cut rounds in half. Place shells 2 inches apart on prepared cookie sheets. Using hors d'oeuvre cutters, miniature cookie cutters or knife, cut out shapes in decorative pattern from shells.

For heads, cut 12 circles with 1-inch round cookie cutter. Moisten back of head and place at bottom center of shell; press down. Cut remaining dough into 1/2- to 3/4-inch circles; cut circles in half for feet. Place feet on prepared cookie sheet near left and right edges of shells; press together to seal. Decorate faces with candies as desired, or leave plain to decorate after baking. Generously fill cutout shapes in shells with crushed candies.

Bake 8 to 10 minutes or until edges are lightly browned and candy is melted. Remove foil and cookies to wire racks; cool completely. Decorate faces and shells with assorted icings, candies and decors as desired.