Yes, you can absolutely substitute triple sec for grenadine and it is excellent to use if you’re in search of a non-alcoholic alternative for your guests that would prefer a mocktail over a cocktail.
That being said, they do differ quite a bit in looks and in flavor. While triple sec is an orange-flavored liqueur that has a sweet citrus flavor, grenadine is characterized by its sweet and tart taste that comes from refreshing pomegranate juice. Alongside this, triple sec is a colorless liqueur, whilst grenadine is known for being red in color that adds a splash of color to any cocktail.
However, it is worth mentioning that the main difference between the two is that triple sec is alcoholic, whereas grenadine is alcohol-free. This makes grenadine perfect for individuals who don’t like the taste of alcohol or who are simply t-total. On the other hand, if you’re looking for another alcoholic substitute for triple sec, then Cointreau is often used as a different alternative, too.