Why do some of the tasks are marked in red, and what is the meaning of different colors and shapes?

The diagram uses a simple color coding to facilitate the perception of information. The project, tasks, and subtasks are all in different colors. Additionally, the intensity of the color and shape of the segments' ends also carries meaning. The red marks overdue task, while light red indicates a job done with a delay or conflicting with the terms of parental tasks.

It may sound quite complicated but it is quite easy in practice. Chart's legend is always at hand.