Just read a thread here about prostitutes in colombo You won't discover the prostitution places from the outside as it's illegal and often disguised as some kind of health massage salon.
If you don't like areas with too many drunks and seedy people on the street, avoid Galle Road segment in Kollupitiya just south of Fort as that's where the casinos are.
There are no specific danger areas as such. Colombo is quiet at night except for few areas that have clusters of restaurants and hotels. For you to come across a "dangerous area with high crime rates" then you'd have to actually go looking them down alleyways and slums.
That and shady nightclubs Read it here tripadvisor. Yeah but those areas aren't so bad either. Casinos are meant for foreigners so the area is monitored by security etc.
I'm not sure exactly which segment of Galle road that Erik is referring to. I doubt that's relevant to you. The original post in the link you provided has been removed. From the couple of responses, it seems that it wasn't appropriate for the forum.
