You might also want to look at Chennai as your entry point to India. Some nice sightseeing in the surrounding areas such as Mahabalipuram (if temples float your boat!) and easy to get onward flights to Colombo.
I was looking for availability myself to Colombo for next year -it was much cheaper in cash/Avios terms to fly to Chennai and get a LCC from there for the 1 hour flight to Colombo. There seemed to be decent reward availability to Chennai too.