Divide the white chocolate chips evenly among 3 microwave-safe bowls, using 1 1/2 cups in each, then add one-third of the condensed milk to every bowl. The portions should look evenly split so the colored layers stack at the same height.

Line an 8x8 baking dish with parchment paper and leave some overhang on the sides. The paper should sit flat in the pan with enough extra at the edges to lift the chilled fudge out cleanly later.

Heat the first bowl for 30 seconds, stir well, then microwave it for another 30 seconds and mix until smooth before stirring in eight drops or more of blue gel food color. When it is ready, the mixture will look glossy and spreadable with an even blue color and no dry streaks.

Spread the blue mixture into the lined dish and smooth the top with a spatula, then refrigerate it for 10 minutes. The layer should look level and lightly firmed so the next layer sits on top instead of sinking in.

Microwave the second bowl for 30 seconds, stir, then heat it for another 30 seconds and stir until smooth before spreading it over the chilled blue layer. The white layer should glide across the surface without dragging too much blue into it, then chill it for 10 minutes.

Warm the third bowl for 30 seconds, stir, then microwave it for another 30 seconds and mix until smooth before adding eight drops or more of red gel food color. The finished mixture should be creamy, evenly tinted, and easy to spread without looking oily or curdled.

Spread the red layer over the white layer, level the top, add red sprinkles, and refrigerate for 10 minutes. The top should set just enough to hold the sprinkles and keep the red layer distinct.

Take the pan from the refrigerator, let the fudge stand briefly at room temperature, then cut it into squares with a very sharp knife. The slab should feel firm but not rock hard, which helps the knife pass through the layers without cracking them.