Chill the mixing bowl and beaters in the refrigerator for at least 30 minutes and line a 9×5-inch loaf pan with plastic wrap.

Whip 2 cups cold heavy cream on medium-high speed until medium-firm peaks form, about 3–5 minutes; in a separate bowl stir 14 ounces sweetened condensed milk with 1 teaspoon vanilla.

Gently fold one-third of the whipped cream into the condensed milk to loosen, then fold in the remaining cream in two additions using a cut-and-fold motion until just combined.

Divide the base into three nearly equal bowls (about 1 1/3 cups each). Add red gel color or reduced strawberry purée to one bowl and stir gently; add blue gel color or reduced blueberry purée to another bowl and stir gently; leave the third bowl plain for white.

Spread a thin 1/2-inch layer of the red mixture into the prepared pan and smooth; freeze uncovered 15 minutes to set slightly, then add a 1/2-inch layer of white and chill 10 minutes, then top with the blue mixture and smooth.

Cover the pan tightly with plastic wrap pressed onto the surface and freeze at least 6 hours or overnight until firm.

Let the pan sit at room temperature 5–10 minutes before scooping or dip your scoop in hot water for easier, clean scoops. Serve with fresh berries or crumbled shortcake if desired.