Coca Cola is brown, that is the colouring. Around 20 yrs ago my parents used to work at a Wimpy in Brighton. Coca Cola actually came in large vats of brown slush. So it had to mixed with soda water to make the drink. You could actually choose the strength as well.
Before they put the colour in - burnt suger - it looks like sligtly murky clear liquid. With the colour in it looks likes brown sludge. PHOSPHORIC ACID (the stuff they clean metal urinals with) is then added to make it see through!