When you navigate to Projects > Planning, some projects may be preceded by a green or red button as shown in the screenshot below.
These colours are an indication of the health score per project.
Red means that at least one phase is overdue, and the deadline of the project may be jeopardized. If you click on the red button, you'll see the names of all overdue phases.
A green colour means the project is running smoothly and no phase is overdue. If you click the green button, you'll see a notification stating the project is being carried out according to plan.