We can answer this question at once if we know the answer to the question, “Why are dark things dark?” A thing is dark, even in light, because instead of reflecting the light from its surface, it absorbs all the light rays. Light and radiant heat are essentially the same thing. So, when light-colored clothes throw back the light that strikes them, they also reflect the heat that accompanies the light rays. Probably nothing will reflect all the light and heat that strike it, and even the whitest snow will gradually melt under the Sun’s rays.
But while light things keep only a little of light and heat that fall on them, dark things absorb practically the full consignment they receive and so, of course, they get warm. Thus a shirt of white cotton, for instance, will become really warmer to wear if it is simply dyed black.