Folks have been very helpful re my several questions in preparation for our recent 10-hour transit through Colombo, which included leaving our bags at the airport when we went into town. Since I have some specific experience now with the left luggage counter, I thought I'd share it.
The left luggage counter (though it may go by some other name) is located outside the doors of one of the two terminals. (Perhaps this is the departure terminal--in any event, as you face the terminals, it is the one to the left and within easy walking distance of the other one.) When we approached the counter no one was staffing it. But, as was our experience, the information desk next door to it handles left luggage if the LL counter is not staffed.
The information counter (and LL counter, when staffed) is open 24 hours. The fee is pretty nominal--something like R100 or R200 per bag to leave it for the day.
Actually leaving our luggage for a 10-hour stopover was not the easiest process. For one thing, the counter personnel require that any left baggage be secured with a lock that you must supply yourself. I suppose this is to make sure they can't be accused of stealing anything, and perhaps for security reasons. They also require that before the bags be secured with locks, a security representative inspect the interiors. We then carried the bags ourselves into the room where they are stored, and were issued receipts for them that noted the tag numbers the counter personnel attached to them.
Since we had four bags we wanted to leave and only two locks, we had to do a lot of cramming and repacking on the somewhat hot, crowded sidewalk by the counter, and even then we had to take our mostly empty smaller bags into town with us. So folks in a similar situation might want to put in or take out any items when still in the comfortable arrive hall, rather than at the counter. (But were able to leave them with the bell desk of the Galle Face Hotel, where we had dinner.) Ironically, inside the (departure) terminal where the counter is located, there is a store that sells all sorts of travel goods, including locks. But we did not know that until after we picked up the luggage.
Retrieving the bags from the counter when we arrived back at the airport was not a problem.