Kathmandu is 0.25 hours ahead of Colombo
Best overlap: 8:00 AM to 6:00 PM in Colombo is 8:15 AM to 6:15 PM in Kathmandu.
8:00 AM to 6:00 PM in Colombo is when Kathmandu is also working, at 8:15 AM to 6:15 PM their time. The 0.25-hour gap barely gets in the way — you've got 10 hours of overlap, so pick whatever time fits the agenda.