Colombo is quite crowded and the traffic can be bad. However, if you know which times to avoid you can actually get around without too much trouble. There is a public transport system; buses and trains but these really aren't that great unless you're travelling to the coast or outside the city. So hail a tuk-tuk (preferably use an app Uber works fine, there are also local versions of it such as Pickme – it's much cheaper and more reliable) nothing completes a trip t South Asia without a tuk-tuk ride. If you meet a friendly driver you could probably try to drive it also.