Well the reason that some clothes are dark is because the properties of the material or dye cause it to absorb more of the light it interacts with. Lighter colored clothes reflect back most of the light. It is the same reason that when you look up at the night sky it is black, because the earth is facing away from the sun during the night and there is very little light except from distant stars entering your retinas. However unlike your eyes which are sensitive to light and use the light entering your eyes to see things, your shirt just heats up with the light it absorbs which isn't very pleasant on a hot summer day.
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Also as with light/dark, certain colors mean that your clothes absorb more light. The color of the shirt means that it is reflecting that color light into your eyes which is how your eyes perceive all colors. The colors to the left have a smaller wave length and therefore carry more energy. So for example if you were to wear a red shirt, it should be cooler than a blue shirt.
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So yeah sorry for getting so nerdy but I definitely agree and can feel the difference first hand when I wear a black shirt or pat my dog when she's standing in the sun (she has black fur).