Prepare the pomegranate syrup by gently heating together the sugar and water in a small saucepan until the solution begins to boil. Remove the pan from the heat, add the pomegranate juice and leave to cool, then refrigerate until ready to use.
To prepare the green sugar for the rim, put the sugar into a food processor and add a few drops of food colouring, then pulse the sugar until the desired colour is reached.
To assemble the cocktails, pour in a little of the pomegranate syrup into a small saucepan or bowl and tip the green sugar onto a plate. Lightly dip the rims of 4 tumblers into the syrup, then dip the syrup-coated rims into the sugar. Spin the tumblers so that the entire rims are covered.
Pour the Cava into the glasses to a level of 1.5cm below the sugar rim. Add 2 tsp of the syrup and top with the pomegranate seeds.