Why- do you walk to Colombo?
Take a train, go to the AA near the Galle Face Green, show them your international drivers licence, pay about 3000 rupees, get a Valid Sri Lankan Foreigners drivers licence- lasts about a year. That is what I did.
My husband went to the central licensing centre in Colombo. You pay about 20 thousand rupees and you get your sri lankan licence in about half a day. It lasts at least 10 years. Take your passport and german drivers licence.
Only two ways I have heard of. Goodluck.