I did a bit of research on the caramel color and this is what I found:
Carmel color is bitter, tastes like burnt sugar and is acidic. It is actually so acidic that bacteria won't grow in it. It is the world's most used food coloring. Food and beverage manufacturers use it to protect their flavorings from oxidation and color their food or beverage.
Here is an interesting video: Secrets of Caramel Color - YouTube
Here is a Wikipedia article: Caramel color - Wikipedia, the free encyclopedia
Perhaps our juice vendors are using caramel coloring in place of or in addition to lemon or vinegar to lower their juices PH?
Anybody have ideas?