Pistachio ice cream is green because pistachio nuts are green. There are lots of pistachios in pistachio ice cream. So when that vibrant green pistachio paste is combined with white ice cream a light green color is formed. Many pistachio ice creams have green food dye added to them to make it look even more green. Depending on the ratio of nuts to dairy the color may be lighter or darker. Recipes that don't use as many pistachios will be paler so food coloring is added for a big green boost.