Move the sherbet to a punch bowl with a spoon.
Carefully pour Sprite into the bowl - don’t freak out: it will foam and bubble!
Add the food colouring, slowly stirring and observe how the colour changes. Stop when it’s green enough.
Serve in the punch bowl or pour into individual bottles/vials.
The foam tends to dissolve over time. I left it in the fridge overnight and the next day it was gone, as you can see on the photos. if you would like to have it foamed at the party, just make it right before it starts.