Islamabad is 0.5 hours behind Colombo
Best overlap: 9:00 AM to 6:00 PM in Colombo is 8:30 AM to 5:30 PM in Islamabad.
Shoot for 9:00 AM to 6:00 PM in Colombo; your Islamabad colleagues will see that as 8:30 AM to 5:30 PM. That's a roomy 9 hours of common time each day, so finding a slot that suits everyone is rarely a problem.