Within every project, there is a Project info tab in the header. Click on it to find the "Progress" section with the subcategory "Roadmap". This article explains the color coding used in that roadmap.
Red: The deadline has passed but the phase hasn't been closed yet.
Blue: The deadline hasn't passed, and the phase hasn't been closed yet.
Green: The phase has been closed.
'Phase' in your project overview
When you return to the general overview of your projects, you can also add the column 'phase' to that overview via the cogwheel in the bottom right-hand corner. If the colour of your phase turns orange, then it means that the due date of your active phase has passed.