Buses leave for Colombo from the Negombo bus terminal about every half an hour. The schedule is similar the other way round. A non AC bus costs 53 LKR per seat and AC bus costs 100 LKR. These two cities are only 36 kilometers apart, but due to the traffic the travel can take up to 2 hours by road. When going from Negombo to Colombo, the bus slows down considerably as it gets closer to destination. On the rush hour journey from Colombo to Negombo, the bus is extremely crowded and the whole journey is very slow due to the traffic. So the best times to get on a bus would be before 8:00AM and between 2:00PM to 5:00PM. I consider this the most convenient public transportation option between Colombo and Negombo. The newly renovated Negombo bus terminal is cleaner place compared to its counterpart in Colombo. As such, this terminal doubles up as a decent resting place.
There are two trains leaving Negombo, at 6:00 in the morning and at 10:00. There is no first class seats, only seconds and thirds.
In theory, it should also be possible to travel between Colombo and Negombo by boats or ships. But this remains an unexplored avenue to me for the time being.