Pour the grenadine syrup into a highball glass, then fill it with ice.
In a cocktail shaker, add the light rum, pineapple juice, and a few ice cubes. Shake well, then slowly strain over the ice in the glass.
Add the yellow pineapple rum and blue curaçao in a cleaned shaker with a little ice. Give a light shake and carefully strain over the back of a spoon on top of the layered drink.