The darker colas can also contain caffeine. The ones that come from the coco bean, and that is not allowed on the program. Any soda that contains caffeine can have non-desired results. Pop in moderation can be acceptable for the average person. You also get into the issue of regular sodas being pretty much all sugar, so that can be a source of empty calories for people.
Simple sugars can also raise triglycerides.
Water is probably the most preferred fluid. I don't think many Americans drink enough water a day. Your typical recommendation would be eight eight-ounce glasses of water. That's calorie and caffeine free, and is actually a nutrient your body needs. It's the most preferred fluid.