Cause Diet dr. Pepper has a lighter color. And Dr.pepper has a darker color than diet dr.pepper. Cause like if you where black outside and it would be like summer.You would get hot fast. Trust me I had to be in black a hole football season.
Things added and in particular the sugar or artificial sweetner lower the freezing temperature of water(soda). Artificial Sweetners take smaller quantities than sugar to accomplish the same feat. More you add the lower the temperature to freeze.