Both destinations are served by Katunayake (CMB). Since there's no flight route between them, the best way to travel is by road.
Colombo
Fly to Katunayake (CMB), then travel by road to Colombo. Taxis, buses, and private cabs are available from the airport.
Thiruvananthapuram
Fly to Colombo (CMB), then travel by road to Thiruvananthapuram. Local transport options include taxis, auto-rickshaws, and state buses.