The Gantt schedule will not color fill a cell if there is no participant costs associated with the corresponding time period. This is actually an error catching feature to ensure that only time associated with the project cost time line are displayed on the schedule.
This behavior will therefore occur if there are zero costs per hour for project participants. The solution is to edit the Human Resources section and apply a cost per hour to each participant. A rudimentary cost can be used for this purpose (such as 1) if there is no real cost required for the analysis.
