If you want a cheap flight from Colombo (Sri Lanka) to Singapore, you've got zero airlines to choose from.
Make sure you use the search options above to search for flights to other airports in Singapore, not just Singapore Changi International Airport. The more options you look at, the better your chance of finding a cheap deal.
If you're planning on going to Singapore during this time, you could take a look at other airport options. Or you could try to get a last-minute bargain.