In a small saucepan combine the high fructose corn syrup (or simple syrup) with the caramel color and the phosphoric acid solution (or citric/phosphoric acid substitute). Warm gently over low heat while stirring until well combined and slightly thickened, about 2–3 minutes. Do not boil. Remove from heat.

While the syrup is still warm, stir in the vanilla extract, orange oil (or orange zest infusion), lemon oil (or lemon zest infusion), and a pinch of cinnamon if using. Stir until homogeneous. This is your cola concentrate.

Assemble Pepsi-style Soda

Chill a glass and fill it with ice cubes (about 1 cup or to taste).

Pour 2 cups of well-chilled carbonated water into a separate measuring container. Gently add the prepared cola concentrate to the carbonated water: start with the full prepared concentrate from the 'Prepare Syrup' section (this recipe is scaled to about 2 cups finished soda). Stir gently once or twice to combine. Avoid vigorous stirring to preserve carbonation.

Taste and adjust: if sweeter cola is desired, add an additional 1/2–1 teaspoon of syrup (or 1–2 teaspoons simple syrup). If brighter citrus is desired, add an extra drop of orange or lemon oil. If more acidity is desired, add a tiny additional pinch (1/16 tsp) of citric acid, or a few drops more phosphoric acid solution. Adjust only in very small increments and mix gently between adjustments.

Serve immediately in the chilled glass. Garnish with a lemon or lime wedge if desired.