Dip the mint or sage leaves in boiling water for 2–3 seconds, then rinse them under cold water. Pat the leaves dry with paper towels and chop them finely – there should be about a tablespoon.
Pour the apple juice stock into a large, heavy-based saucepan, add the sugar and stir over low heat until all the sugar has dissolved. Bring the mixture to a full rolling boil and boil for 5–10 minutes. Remove the pan from the heat and test if setting point has been reached.
When the jelly will set, quickly skim the scum from the surface. Stir in the chopped mint or sage and a few drops of green colouring, if using. Pour into clean, warm jars and cover immediately with waxed paper discs and vinegarproof screwtops or cellophane covers. When the jelly is cold, label and date the jars and store them in a cool, dark, airy cupboard.