I took quite a bit of time to actually research and study about this particular urban myth and as far as I can tell, I was not able to find any official documentation about this.

However, what I was indeed able to find was official wording that made clear that Lee Kuan Yew, during his visit in 1956, spoke very highly about Colombo. He thought Colombo was a much better planned city than Singapore and he initially thought very highly about Sri Lanka's prospects as an independent nation, lavishing praise on Sri Lanka's education system, civil service, and its access to financial reserves. But on the other hand, he also made very similar remarks about a number of other Asian cities as well during this time period of de-colonization and post-independence growth.

This was actually talked much about in his memoir "From Third World to First".

However, I did once made a small-scale research project, out of personal interest, of Singapore's history of early urban planning and post-independence city-planning, and I saw no reference whatsoever that any such planning was modelled on Colombo.

I would be curious, to know more about this urban myth, like how long has this urban myth been around and if it is widely believed.