Catch bus number 98 from Colombo Fort private or public bus stations. Some buses travel from Colombo to Pottuvil, whereas some go on up to Panama. One of the easiest ways to go from Colombo to Arugam Bay, catch a ride in either of them and you will be only 8 hours away from the cooling breeze of Arugam Bay. Sudden traffic, especially during rush hours might affect the travel time.
If you are not much of a heat-person, then this is for you! The A/C bus can be boarded from the Colombo Fort Private Bus Station. This bus goes from Colombo up to Akkaraipattu. Next, you can catch a bus to the Pottuvil town from the Akkarapaitu bus station at any time of the day. Finally you will have reached Arugam Bay.