Green Chartreuse was the first thing that came to mind. It’s kind of a yellow-green, but the greenest liqueur I can think of. I have a bottle but haven’t experimented too much with it aside from a couple Last Words. Last Words come across more yellow than green if I remember correctly.

Creme de Menthe comes in a green variety, but you’re basically limited to Grasshoppers. I don’t remember my parents drinking much (or really at all), but my mom would bring me to a liquor store once every few years in March to buy Creme de Menthe. She wouldn’t make drinks with it, but use it for baking things such as grasshopper pies.