In a 4-quart Dutch oven combine rhubarb, sugar, water, and strawberries. Bring to boiling; reduce heat. Simmer, uncovered, 10 to 15 minutes or until fruit is very tender, stirring occasionally. Reduce heat. Add marshmallows and lemon juice. Cook and stir until marshmallows are melted. Transfer to a very large mixing bowl. Cover and chill in the refrigerator 4 hours or overnight until completely chilled.
In a chilled large mixing bowl beat whipping cream with an electric mixer until soft peaks form. Fold whipped cream into chilled fruit mixture. Stir in enough food coloring to reach desired color, if desired.
Freeze ice cream mixture in a 4- or 5-quart ice cream freezer according to the manufacturers directions. Transfer to a freezer container. Cover and freeze about 4 hours or until firm. Makes about 10 cups (twenty 1/2-cup servings).