In short, dark colors absorb more light, which is a kind of energy that turns into heat. That's why black color absorbs more heat.
For the detailed process, there are two steps:
- The color you see is the color that is reflected from an object. A blueberry reflects blue light; therefore, making the blueberry…blue. A red grape reflects red light making the grape red. A black opal does not reflect any light, absorbing all colors, therefore making it black. A white piece of paper reflects all light and absorbs none making it the color white.
- The light contains heat in itself. When the light energy is absorbed by an object, the light energy is released as heat energy. The more light a color absorbs, the more heat it will radiate. As dark colors absorb more light and light colors absorb less light, the black color absorbs more heat, either. White, on the other hand, does not absorb light; but rather, reflects all light, so does the heat.
Try laying out in the sun on a white towel. You will actually feel fairy cool. However, as some I know have done, try laying on a black roof to sun bathe, and you will feel burnt before you even really are because of the black absorbing the light and radiating more heat.