As far as your question about currency, if you're not wanting to exchange to their local currency then US$ work fine in the airport, but I wouldn't expect the same if you go to local places outside. Colombo is actually a pretty cool place and would be well worth a visit on a 16 hour stopover if you have some useful daylight hours in there, but it's not a cheap city nor is transportation to/from the airport. Aside from that, the suggestion of a cheap airport hotel is probably best, 16 hours in the airport itself would be pure torture, it's small and very little to do to pass so much time.