Pour the grape juice into a small saucepan and bring it to a gentle simmer over medium heat.
Simmer for 8 to 10 minutes, stirring occasionally, until the juice reduces by about half. You should have about 1 cup of grape reduction.
Transfer the grape reduction to a heat-safe bowl and cool completely before using.
In a large bowl, whip the cold heavy cream until soft peaks form.
In a separate bowl, stir together the sweetened condensed milk, cooled grape reduction, vanilla extract, lemon juice, salt, and optional food coloring.
Fold a small amount of whipped cream into the grape mixture to lighten it.
Gently fold in the remaining whipped cream until smooth and evenly combined.
Transfer the mixture to a loaf pan or freezer-safe container and smooth the top.
Press parchment paper or plastic wrap directly onto the surface, cover tightly, and freeze for at least 6 hours or overnight.
Let the ice cream soften at room temperature for 5 to 10 minutes before scooping.
