Expect purple rather than black
This punch gets its color from juice, not a black coloring ingredient. Keep the measured grape and cranberry quantities rather than adding more juice until it looks like a photograph. Combine them with the lime first and leave the soda sealed until serving. Chilled ingredients help the drink reach the glass without needing a long wait beside melting ice.
Put out all four glasses before adding the soda. Divide the rinsed berries between them so one serving does not get most of the fruit from the bottom of a pitcher. Stir gently and serve promptly. If planning for guests with ingredient restrictions, keep the actual juice and soda labels available; a dark color does not tell you what sweeteners or other ingredients a product contains.
