The reason green bean soup sometimes appears red is because during the cooking process, a component within the green beans called anthocyanin combines with acid to produce a red pigment. This is more likely to happen in an acidic environment, such as when acidic seasonings like lemon juice are added.

As for whether red or green is better for green bean soup, it's a matter of personal taste. Traditionally, green bean soup is predominantly green and is considered to be a refreshing and cooling drink. However, some people prefer red green bean soup, believing it to be more visually appealing.

Both red and green green bean soup are similar in flavor and nutritional value. The green bean itself is rich in fiber, protein, vitamins and minerals, and has the effect of clearing heat and detoxification, diuretic and swelling. Therefore, there is no particular good or bad points in the color, the choice of which color of green bean soup depends entirely on personal preference.