My wife and I recently decided to purchase a few shipping containers to turn into a workshop. We haven’t bought the containers yet, but I’ve already started to think about possible color schemes.
I’m thinking about colors this soon because I know the containers will need to be painted. The original colors we end up with will likely be random based on what’s available at the time, and the containers probably won’t match each other – we do live in the high desert, after all, hundreds of miles from the nearest coastline; so we can’t be picky when it comes to getting shipping containers out here.
More importantly, painting the containers is necessitated by the fact that one of the biggest problems with a shipping container is that it’s a giant, steel box that gets extremely hot in direct sunlight. It is essentially an oven. Choosing the right colors plays a huge role in dictating how hot, or not, the containers get.
Many variables influence how much radiant heat an object can reflect or absorb. Some of these variables I will discuss before we get to the colors. But just keep in mind that the material, density, shade/tint, specularity, and even the shape can have large impacts on heat absorption and retention.
In other words, just because one object is red, a similar object is blue, and blue technically draws more heat than red; does not mean the red object can’t get hotter than the blue object due to other circumstances and variables.
Light vs. Dark Shades of Color
No matter the color, the darker the shade of that color, the hotter it will get and vice versa – i.e., dark red will absorb more radiant heat than light red, and so on. So when choosing the right color, remember that the hue and shade, or tint, are both important.
It’s also worth mentioning that an objects specularity, or reflectivity, can have an impact on how much heat it absorbs or reflects. A glossy surface will reflect more heat than a flat surface.
Think about looking at your phone’s screen while it’s turned off: even though the screen is black, you see your reflection, like a black mirror – I suspect this phenomenon inspired the name of the popular Netflix show, “Black Mirror.”
Point is, if you can see your reflection, a lot of light is being reflected back at you, which means none of that heat is being absorbed regardless of the color.
It’s Not Always About Staying Cool
In the southern United States, we benefit from colors that stay cool in sunlight; however, in cold, cloudy climates, you’re better off choosing colors that absorb lots of radiant heat rather than reflect it.
No Color Is a True Radiant Barrier
Although some colors reflect more heat than others, none of them are radiant barriers – not even white. That is because every color absorbs some amount of visible light and a lot of invisible light – e.g., ultraviolet and infrared.
If you’re looking for a true radiant barrier, you need a highly reflective metal, such as shiny aluminum, silver, and gold; or a material that has been aluminized. Even so-called “radiant barrier paints” are not effective radiant barriers, but that’s probably a subject for another article.
Colors from Coolest to Hottest
Understanding why some colors get hotter than others is actually quite simple: When you see the color of any given surface, what you’re seeing is the wavelength of light that is reflected by that object. Longer wavelengths, like red and orange, are hotter than shorter wavelengths, like blue and violet.
So an object that is a “warm” color will reflect the hotter wavelengths whilst absorbing the cooler wavelengths and vice versa.
If Warm Colored Surfaces Don’t Absorb as Much Heat, Then Why Do Hot Objects Glow Red, Orange, and Yellow?
When you see a stove coil glow red, or a bright flame glow orange, you are not seeing light being reflected, you are seeing light being emitted.
So, with all that in mind, let’s now go through all the visible colors from coolest to hottest.
Though technically not a color, a white surface stays cooler than any other hue. That is because pure white reflects all visible light, which is also why it’s not a true color – all colors are being reflected away. So if all the wavelengths are being reflected, then none of the energy from them is being absorbed. Keep in mind however, that white does absorb ultraviolet and infrared waves, so it will still heat up to some extent.
In art class we are taught that red is the “warmest” color, and while it does invoke thoughts of heat and warmth, when we’re talking about the color of a surface, this is somewhat backwards. Red light wavelengths are warm but red surfaces are typically cooler than other colors.
Red is the longest visible wavelength, so red objects actually absorb less heat energy than any other true colors of the same shade or tint because the warm red wavelength is being reflected away from the surface.
Orange is the next color after red on the visible light spectrum, and similarly has a pretty long wavelength. So even though it reminds us of fire, it doesn’t absorb nearly as much heat as you might assume.
Yellow naturally follows orange, and is also on the long, or slow, end of the spectrum.
Green is where things start to heat up a bit. Green sits right in the middle of the spectrum, with a medium wavelength. It’s not exactly great at reflecting or absorbing heat. Nevertheless, expect green surfaces to typically get much hotter than red, orange, and yellow.
Cyan is where we start to enter the short end of the spectrum. Cyan reflects its relative short, cooler wavelength while absorbing more of the longer, hotter ones.
Once we enter the blue field of the spectrum, frequencies start to get very high. If you want to ensure that something gets proper hot from the Sun’s rays, a dark shade of blue with little to no sheen is guaranteed to do the trick.
Violet has the shortest wavelength and thusly sits right at the end of the visible light spectrum, which means it has the greatest potential for heat gain of all the colors.
Black, like white, is not a true color. Rather, it is all of the colors being absorbed on one surface, so a black object will get much hotter than any identical object of a singular color.
Suppose we have two shipping containers that are identical in every way, except one is painted black, and the other is painted any color of the rainbow. The heat absorbed by the black container will absolutely dwarf the heat absorbed by the colored container.
Take a look at these results from an experiment conducted by Coerco. They wanted to see how hot polyethylene tanks of various colors would get under the same conditions.
As you can see from my screenshot below, the black tank absorbed far more heat than any other. What else is interesting is how the black tank – as well as the other dark colors – rose in temperature disproportionately to the increase in ambient air temperature, and vice versa for the light colors.
An 8ºC temperature increase caused the black tank to get more than 10ºC hotter. On the other hand, the yellow and white tanks did not rise in sync with the hotter weather. Those tanks only gained about 5ºC.
In conclusion, the colors with the shortest wavelengths get hotter than the colors with longer wavelengths. Contrary to common belief, red and orange will actually absorb less heat and thus, will keep your surfaces cooler, all things being equal, compared to violet and blue.