
Walnut liqueur is a sweet liqueur flavored with walnut, most often from unripe green walnuts macerated in spirit.
Green walnut steeped in spirit is not something a shelf substitute reproduces — the tannin and the bitterness under the sweetness are the point. Another nut liqueur gives the color and the sweetness and leaves the drink rounder than it should be.
Made from roasted ripe nuts rather than green ones with their husks; sweeter and rounder, and the drink loses the bitter green edge it was built on.
It is the unripe green fruit, husks and all, that gives the near-black color.