Melt white candy melts in microwave according to package directions; stir to smooth.

Transfer ? cup each of melted white chocolate to two small bowls. Add red and blue food coloring to one bowl and green food coloring to the other, stirring as needed to achieve purple and green colored candy. Reheat in microwave if necessary to thin out consistency.

Lay cookies on a baking sheet or 9x13 baking pan lined with parchment paper. Pour white melted candy over cookies and smooth with rubber spatula.

Drizzle colored melted candy all over white candy; immediately add sprinkles evenly, followed by M&Ms, candy pumpkins, and other Halloween candies and decorations of your choice.

Place in fridge or freezer until solid; use a sharp chef's knife to cut into bite-sized pieces.