Referring to the wearing of dark colors in hot climates...

one of the main reasons dark colors are worn is that they create thermal updrafts beneath them. The burka, for instance, a full body robe, when worn in the sun, will heat up as the sun shines upon it. the air directly beneath the cloth will also heat up and rise, to exit at the neck opening. This will create circulation beneath the garment, thusly keeping the person wearing the garment cooler.

does this apply to a kilt or belted plaid? I do not think so, as they are "sealed" at the top by a belt, preventing the warm air from escaping. this is one of the reasons wool kilts are so warm in the winter, as they trap the warm air within them.