I always thought the pink version of the Fanta tonic water had bitters in it. That's a common drink -- gin, or rum, and tonic with bitters.
Pink Gin (gin, typically Plymouth gin) with tonic water and a couple of dashes of Angostura bitters is a classic drink in England.
But bywarren is right -- some Belize Fanta tonic water is clear and some is pink. Bartenders sometimes give you soda water when you ask for tonic water, and vice versa.