Darker colors absorb more light. Since light is energy, absorption would increase a material's temperature. This means that, darker colors become better radiators of heat. It is important to note that an object appears white if it reflects all colors and black if it absorbs all colors.
A black object absorbs all wavelengths of light and converts them into heat, so the object gets warm. A white object reflects all wavelengths of light, so the light is not converted into heat and the temperature of the object does not increase noticeably.