Darcy O'Neil's "Art of Drink" videos cover this:
The Science Behind Crystal Pepsi: Microemulsion
https://www.youtube.com/watch?v=ppufxi1pHu8
>If you've ever asked the question: How do they make Crystal Pepsi clear? Here is your answer. The method is surprisingly simple and uses a technique to make a microemulsion where the droplet size of the flavour oil is around 1 micron. This method comes directly from Pepsi, via a patent they filed in 1989, providing a detailed method for producing oil-in-water microemulsions for beverages. The patent specifically discusses clear colas and methods for producing the same.
>This video is an introduction to creating microemulsions and provides a process for doing it at home with simple equipment and easily available ingredients.
Also:
Emulsion FAQ (Beverages)
https://www.youtube.com/watch?v=GRf4ehRPpok
Stable Flavour Emulsion for Soda Syrups and Non Alcoholic Drinks
https://www.youtube.com/watch?v=lYToDpVAeMU
Caramel Colouring, Flavouring and Emulsifying Syrup