My husband is Jamaican so I feel that I can answer this question just partially as I am not Jamaican. Jamaicans dress the same as Americans, Canadians, or Brits. There is no special “costume”.
Personally, I think they overdress a bit for the heat, but they’re used to the climate. Tourists run around in shorts and sun dresses, but Jamaicans wear regular office clothes, e.g., suits, ties, nice dresses. While I feel quite comfortable in shorts and a tee shirt, my mother-in-law wears a dress with a cardigan sweater. They tend to wear more somber colors, in my opinion.
Children wear uniforms to school. Each school has its own colors.