Common sense should tell you that a dark colored car, especially a black one, will be hotter than a white car. So will direct observation and experiment with a couple of thermometers and two identical black and white cars sitting out in the sun (as the Mythbusters confirmed). The laws of thermodynamics and spectroscopy will also confirm this.
Black will absorb and radiate heat towards whatever is cooler until an equilibrium is reached depending on the heated body's ability to be cooled (like with a fender or hood, by the surrounding air), and white will reflect heat and not radiate the heat well at all. Thermal energy, heat energy, has all of its radiation being in the infrared part of the energy wavelength spectrum. Each color of the visible spectrum of light tend to focus their ability to both absorb and dissipate in the wavelength of their color, and far less in the infrared (which is where the heat is) compared to black.
The Mythbusters observed two cars sitting side by side in direct sunlight, with the black car's interior having heated up to a temperature of 135 °F, while the white car topped off at 126 °F. That experiment was done on a day where the high temperature reached 88 °F. Other similar experiments yield similar results, except with higher differences between the temperatures. At 100 °F the interior of a white car can reach 140 °F and the interior of a black car can reach upwards near 170 °F depending on the car.
Here is a chart that shows one of the most visually striking results of the fact that dark colors absorb thermal energy and lighter colors reflect it. Car*Color*Surface*Temperature*Data
The temperatures are external surface temperatures of the roof, taken with an infrared scanning thermometer. Each color on the chart represents a different color of vehicle. Each vehicle was a Prius. Clearly, black is not the color you want on a sunny day. It's also why you see many dark and black cars lately with white roofs, since it's the roof that takes the brunt of a day's sunlight. It's also why it's so important to well insulate your iron box's roof at least or more than the walls.
Because black absorbs thermal energy and radiates to towards whatever is cooler (heat will always move in the direction of cool), even if the metal is 150 degrees, the black paint above it will still be slightly warmer, and the underlying metal will always be slightly cooler than the black paint above, so the thermal energy heat will constantly be absorbed and radiated inward towards the cooler metal. It will continue to do this until an equilibrium is reached, to the point where the black paint and the underlying metal reaches the same temperature.
This works both ways, in that black absorbs and radiates thermal energy towards whatever is cooler, so that when the sun does finally go down, a black car will cool off much quicker than a white car (tho the white car never got anywhere near as hot as the black car in the first place), and will reach an equilibrium with the external outside temperature much quicker than a white car will. For example, if the sun goes down at 8PM, by 10PM the black car's interior may already be the same temperature, or close to it, as the outside ambient temperature, while the white car may require 1 or 2 more hours to reach the same temperature.
This is why you don't see a lot of engines painted white. They're painted black or dark red, or green, or even CAT yellow, or some other dark color, because that dark color absorbs and radiates heat and allows the engine to run cooler, whereas a white engine would reflect the heat back into the engine.
It's also why, under certain conditions, black clothing can be much cooler in the sun than white clothing. Mostly it depends on the wind. The wind needs to be at least 3 m/s, or about 7 MPH.
Black clothing absorbs sunlight and the heat radiating from your body, but if it is loose-fitting, and there is enough wind, the wind convects the heat away faster than it is absorbed. White clothing reflects sunlight, but also reflects internal heat back towards your body, so the net effect under identical conditions is
less cooling than if you wore black.
Note the large number of arctic animals that are fluffy and white. It's not just camouflage. Cold and windy conditions as you find in the arctic, the white and fluffy helps the animals retain heat. Granted, you don't often find fluffy black animals in deserts, and you don't find many white animals, either. Mostly you find animals that blend into the background. Clearly, if heat gain and camouflage are in conflict, the need to avoid being eaten outweighs other considerations. On the other hand, desert-dwelling nomadic people such as the Tuaregs (nomads of the Sahara) wear loose-fitting dark blue or black clothing, and have been doing so for a very, very long time. They are also known as the "Blue Men of the Sahara" in reference to the royal blue robes they wear. If there were an advantage to wearing white clothes, you'd certainly expect they'd have figured that out by now.
Here's Prius*Car*Interior*Temperature*Experiment another chart that is quite frankly very ugly (especially one coming from a Texas Instruments engineer), but shows the importance of shade screens and cracked windows, where just a ½-inch crack in both front windows can make a 20 °F or more difference in the internal temperature of the vehicle. Because of the Sprinter's huge windshield, and because I have these really gnarly thermal shade screens (rather than the cheap silver bubble pack or Reflectix, which is certainly better than nothing), I find that if I park the van facing southwest to where the windshield gets the bulk of the sun when the day is at its hottest, the thermal screens deflect most of the thermal energy and the rest of the van absorbs less heat, and the van stays far cooler.
In the winter, using simple black privacy curtains in the windshield and the side windows will absorb heat and make the Espar run less. Also, while tinted windows are dark, sometimes nearly black in some states, they are designed to reflect both UV and the infrared spectrum of light, so while it may seem like tinted windows would make a car hotter, they don't.
