No, you cannot substitute grenadine for crème de cassis. While both are red in color and tart in flavor, they impart very different tastes to a cocktail. Crème de cassis is a type of liqueur made from black currants, while grenadine is typically a non-alcoholic syrup made from pomegranates or sometimes artificial flavors. The flavor of crème de cassis is sweet and tart with notes of smoked cherries, honey and a hint of bitterness, while the taste of grenadine is mostly sweet with undertones of pineapple or orange. Moreover, the texture and viscosity of these two ingredients vary greatly; crème de cassis is thicker than grenadine, which means it has a slower pour rate making it easier to control the amount used in cocktails.