Colombo is 0.5 hours ahead of Islamabad
Best overlap: 8:30 AM to 5:30 PM in Islamabad is 9:00 AM to 6:00 PM in Colombo.
For the smoothest call, aim inside 8:30 AM to 5:30 PM in Islamabad, i.e. 9:00 AM to 6:00 PM in Colombo. That's a roomy 9 hours of common time each day, so finding a slot that suits everyone is rarely a problem.