It is compulsory to obtain resident/student visa to study in Sri Lanka. In order for that you may have to enter to the country with Entry visa. Please note that visiting/ tourist visa would not permit you to commence your studies.
If you require assistance with obtaining a visa to enter Sri Lanka for the period of study at the University of Colombo please contact the International Office of the University of Colombo, leaving a period of at least three months for the processing of the visa.
For more information regarding obtaining a Sri Lankan visa visit official website of the Department of Immigration and Emigration of Sri Lanka.