Prepare each flavored jello in a separate rectangular plastic container, using 1 cup boiling water and 1/2 cup cold water per box instead of the package liquid amount. Refrigerate for at least 4 hours, preferably overnight, until each pan feels firm and springy when lightly pressed.

Place the unflavored gelatin in a small bowl, stir in 1 cup cold water, and leave it undisturbed for 10 minutes. The mixture should absorb the water and become thick, spongy, and opaque rather than remaining loose.

Add the half and half, sweetened condensed milk, evaporated milk, and vanilla extract to a blender and pulse until smooth. The mixture should look evenly creamy, with no streaks of condensed milk clinging to the sides.

Microwave the bloomed gelatin for 30 to 40 seconds until it returns to a smooth liquid, then add it to the blender and pulse again. The milk gelatin should be fluid and fully blended, without visible gelatin clumps.

Lightly coat the inside of a bundt pan with nonstick cooking spray. A thin, even coating helps the finished dessert release without leaving greasy patches on the surface.

Turn the firm colored gelatin slabs onto a large cutting board and cut them into 1-inch squares. The cubes should hold their corners and lift cleanly from the board instead of squashing or tearing.

Pour a thin layer of milk gelatin into the bundt pan, add a mixed layer of colored cubes, and repeat until all the components are used. Pour gently and distribute the colors around the pan so the pattern looks balanced after unmolding.

Refrigerate the assembled mold for at least 4 hours, preferably overnight. The dessert is ready when the top feels firm and the milk layer does not shift when the pan is gently moved.

Loosen the outer edge and center tube with a clean finger, cover the mold with a large plate, and invert it in one steady motion. Lift the pan slowly once the gelatin settles onto the plate, then slice and serve chilled.