For green pistachio ice cream, add 2 to 4 drops green food coloring after the almond extract.
Finely grind in a food processor, but not to a paste:
Transfer to a medium saucepan and add:
Bring just to a simmer over medium-low heat, stirring to dissolve the sugar. Remove from the heat, cover, and let steep for 1 hour. Strain through a fine-mesh sieve into a medium bowl, pressing firmly to extract all the liquid; discard the nuts in the sieve. Stir in:
Refrigerate the ice cream base until cold, overnight if possible, or quick-chill in an ice water bath.
Pour the base into an ice cream maker and freeze as directed. When the ice cream is almost frozen, add:
Transfer to a container with a tight-fitting lid and freeze for at least 4 hours to harden.