Gantt chart customization in eazyBI currently is quite limited, though it is in our backlog for future development. A color of the bar is changed automatically based on the progress of the task. If a task has no completion ratio specified, the bar is blue.
Another option is to use bar chart range option that will show bar between two dates. See an example report in eazyBI demo account.
In your described case you would specify new calculated members showing start date and end date if the target end is within next thirty days for drawing first range; then another start and end date for issues that have target end between 30 and 90 days; and the last range for target end being in further future.