Kangaroo Cabs is one of the most reliable cab companies in Colombo (for cars, not trishaws). If you call they generally remember who you are and dispatch a cab reliably. Now it’s a bit easier because they have an app (for Android and iOS). You can also order online or call 112588588, but using the app is a pretty cool way to go.
You can download the app via the following links:
For more info, check out their Facebook page.