Step 1: Line a 9×13 inch (23×33 cm) baking pan with parchment paper, leaving an overhang on the sides. Lightly grease the pan before lining to help the paper stick.
Step 2: In a large bowl, combine the finely crushed digestive biscuits with the melted unsalted butter. Mix thoroughly until the crumbs are evenly coated. Press this mixture firmly into the bottom of the prepared pan, creating an even layer. Use the back of a spoon or a flat-bottomed glass to compact it tightly. Chill in the refrigerator for at least 15 minutes.
Step 3: In a separate bowl, combine 100g of desiccated coconut, 65g of powdered sugar, and a few drops of green food coloring. Mix well until the color is uniform. Gradually add 1/3 of the sweetened condensed milk, mixing until a thick, pliable mixture forms. Spread this green layer evenly over the chilled biscuit base. Return to the refrigerator.
Step 4: In another bowl, combine 100g of desiccated coconut and 65g of powdered sugar. Gradually add another 1/3 of the sweetened condensed milk, mixing until well combined and a spreadable consistency is achieved. Spread this white layer evenly over the green layer. Chill again.
Step 5: Repeat the process for the red layer. Combine the remaining 100g of desiccated coconut, remaining 70g of powdered sugar, and a few drops of red food coloring. Add the remaining sweetened condensed milk and mix until evenly colored and combined. Spread this red layer evenly over the white layer.
Step 6: Cover the pan tightly with plastic wrap and refrigerate for a minimum of 4 hours, or preferably overnight. This is crucial for the layers to set properly and for the slice to be easily cut.
Step 7: Once thoroughly chilled, lift the slice out of the pan using the parchment paper overhang. Remove the parchment paper. Cut into squares or festive shapes using a sharp knife dipped in hot water for cleaner cuts. Decorate with festive sprinkles, candied cherries, or a dusting of extra coconut for a snowy effect before serving.