I have noticed very few rainbows during the nearly 6 years we have lived in Singapore. I don't know if that is because I haven't been paying attention, or if the atmospheric conditions aren't conducive to their creation.
On Saturday afternoon, I happened to look out and see a faint rainbow in between Marina Bay Financial Centre Tower 1 and Tower 3. A few moments later, I noticed the arc continued over the corner of Marina Bay.
Perhaps I simply haven't been looking out at the right time to see rainbows.