Place washed and dried green grapes in a single layer on a sheet pan. Freeze for 2 hours or until completely frozen.
Blend the frozen green grapes, lime zest, lime juice, granulated sugar, and milk of choice in a food processor until smooth, scraping down the sides as needed. Once blended, transfer the puree to a 9x5 loaf pan or airtight container. Store in the freezer while you make the next layer.
Blend the frozen raspberries, granulated sugar, and milk of choice in a food processor until smooth, scraping down the sides as needed. Once blended, transfer the raspberry puree on top of the lime layer and store it in the freezer while you make the last layer.
Blend the frozen mangoes, orange zest, orange juice, granulated sugar, and milk of choice in a food processor until smooth, scraping down the sides as needed. Once smooth, transfer it on top of the raspberry layer.
Use a butter knife or something similar to swirl the flavors together for the rainbow effect.
Cover tightly with plastic wrap or a tightly sealed lid if using an airtight container. Freeze until solid, about 2-3 hours. When ready to serve, let sit at room temperature for a few minutes to soften before scooping.