Drift BnB is an awesome place to stay if you're planning on being in Colombo for a few days. It's located right off Galle Road (the main coastal road in Colombo) and is about a 5min. tuk-tuk ride from Galle Face Green (around 100-150 rupees) and a 7-8min. tuk-tuk ride to the Colombo Fort Train Station (200 rupees). There's not a whole lot in terms of monuments that you can walk to, but the transportation is cheap enough that you can reach pretty much every major sight in Colombo in under 10 minutes. There are, however, a lot of good bars and restaurants in the area, including the world-famous Barefoot Cafe, within a 5 minute walk.
Drift is billed as a B&B but really it's a hotel, which is fine. The rooms are thankfully air-conditioned and have comfy beds, satellite television, and hot showers (though you'll probably be taking cold ones to cool off...it is Sri Lanka after all). One nice thing about Drift BnB is that it's got a youthful boutique feel to it without every crossing over into backpacker territory. The main waiting room has some psychedelic-style artwork and the rooms have bohemian touches like old Cointreau bottles serving as flower vases. The staff speaks perfect English and are really friendly. They even have a map prepared of all the good cafes and restaurants in the area.
All in all, I couldn't have asked for a better place to stay in Colombo. It's central, quiet, and has just the perfect blend of tropical authenticity and modern comfort to make for a really nice stay in the city. I'd recommend it 100%.