We can't able to give customize color in Gantt view.
Color coding of the summary bar is as follows:
* Gray shows that the resource is not allocated to any task at that time
* Blue shows that the resource is fully allocated at that time.
* Red shows that the resource is overallocated
refer : https://doc.odoo.com/6.0/developer/2_6_views_events/views/gantt_view/
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