Colombo is apparently derived from Sinhala name Kola-amba-thota which means "harbour with leafy mango trees". (It has been pointed out by Julius de Lanerolle, in an article in the Journal of the Ceylon Branch of the Royal Asiatic Society, that the first part of this compound, "Kolamba" is a Sinhala word meaning port, ferry, harbour or haven.) Ibn Batuta in the 14th Century referred to it as Kalanpu.
The name was altered by the Portuguese to honour Christopher Columbus.

I stepped my feet in Colombo on 1 May, and some images are here to see.