In Colombo there are thousands or at least it seems like thousands of beige, green and red tuk tuks. Some are metered and others you have to agree a price before getting in.
Some tuk tuk drivers want to take you on a detour to a gem shop or currency exchange where they will get a commission, make it clear that you do not want to do that and choose another driver.
Travelling at commuter time is congested so try to avoid the busy times as it’s likely to be more expensive.
Tuk tuk drivers will drive slowly at the side of the pavement trying to sell you a tour, make it clear if you don’t want want one and walk away.