The use of Formats to highlight activities that need your attention is a key feature of To Do List. The default formats delivered with To Do List are listed below.
These formats can be adjusted by accessing the 'Formats' option in the Customise section on the ribbon.
Click on that button and you will see a form that gives you the opportunity to change the formats used in To Do List to highlight items that are approaching a due date, are overdue, or have been completed or cancelled.
To change the format for a particular warning simply click on that option and then use the colours at the bottom of the form to set the back colour and font colour for the warning. For the Past Due Date Warning you can also change the number of days in which that will be displayed for an activity. It is defaulted to 5 days so that an activity is highlighted in the highlight colour and font if the due date for that activity is within 5 days of the current date.