Add each packet of flavored gelatin to a separate medium heatproof bowl. Add boiling water and cold water to each bowl following the package directions and stir well until the mixtures are smooth. Transfer each mixture to airtight containers that hold at least 2 cups each. Refrigerate until firm, at least 6 hours or up to overnight.

When the four flavored gelatins are firm, slice them into roughly 1-inch cubes. Transfer the gelatin cubes to a glass 9x13-inch baking dish or a 10-inch bundt pan, making sure to mix up the colors throughout. Chill in the fridge while you make the white gelatin.

To make the creamy gelatin, in a clean medium bowl, combine the unflavored gelatin with the boiling water and whisk until smooth. Let the mixture cool for 15 to 20 minutes until just warm, not hot.

Meanwhile, in a large bowl, use a hand mixer to beat the heavy cream on medium speed until soft peaks form, about 5 minutes. Add the sweetened condensed milk and beat until combined and fluffy, about 2 minutes. Add the whipped cream mixture to the cooled unflavored gelatin and gently fold to combine.

Gently pour the creamy gelatin mixture into the flavored gelatin cubes in the prepared dish or pan. Gently fold and stir to make sure the creamy white mixture is evenly distributed and coats the flavored gelatin cubes completely. Cool the mixture on the counter until it’s room temperature, 15 to 20 minutes. Cover tightly with plastic wrap. Refrigerate until fully chilled and firm, at least 8 hours and up to 2 days.

If made in a glass baking dish, slice the chilled dessert into individual squares for serving. If made in a bundt pan, place the bottom of the pan in warm water for 3 to 5 minutes. Run a small, sharp knife around the edges of the dessert to loosen it from the sides of the pan. Place a platter on top of the pan, then invert and remove the cake pan. Slice and serve.