The color of an object depends on the wavelengths of colors reflected from the object. A red apple is red because red wavelengths in white light are reflected and other wavelengths are absorbed. In fact, if a red apple were to be illuminated by light that had no red wavelengths, the apple would appear almost black. The darker the object, the better its emission of heat because it is a better absorber of light. It is really pretty simple. The reason dark colors are dark is that they absorb light instead of reflecting it. Since light is energy, absorbing light makes something hotter.