Think of it like this...
Aluminum heats up easier AND cools off faster than, say, cast iron. This is getting into the specific heat of a material, but think it of it as the specific rate of heat transfer, in either direction. Any thing that is slow to heat up is slow to cool off, all other factors being equal. Anything that heats up quick is quick to cool off. This also assumes things like phase change (evaporation, freezing,etc) aren't happening.
Now, instead of calling it heat, call it energy. Heat is a symptom of how much energy is in a thing. The black surface transfers energy faster than other surfaces. In, or out. In this case, radiant energy. It radiates heat/energy faster, and absorbs radiant heat/energy faster. We feel the black patterns on our shirt heat up faster in the sunlight, but they also expel more of our body heat out to the environment.
Close the hood and the engine is absorbing less radiant energy from the sun, because there's less available, but still radiates heat/energy to anything around it.
But again, all other factors need to be equal. Glossy vs. Matte has an effect, because glossy surfaces reflect more light which is its own form of energy. You can look up the specific heat for any material, and I'm sure something similar exists for colors/surfaces, but I don't know what it is.
That's my rudimentary understanding of the thermodynamics of the thing.
