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