Green, blue, gold, and more: What the different colors of hydrogen mean

Hydrogen is a colorless gas. But if you’ve done any reading about the future of energy, you’ve likely seen it identified by a number of different colors—ranging from turquoise and aqua to green, yellow, and red. So if hydrogen doesn’t have a color, why use the rainbow to describe it?

The answer is (fairly simple): The color denotes how the hydrogen was made. That information is increasingly important as hydrogen becomes a promising energy source in the face of the climate crisis. When burned, hydrogen doesn’t emit CO2, a stark contrast to fossil fuels. That could go a long way to decarbonizing the economy—especially in such industries as transportation and construction.

But not all hydrogen is created equal. There are a number of different methods, each with unique costs and environmental impacts. As various methods have fluctuated in popularity over time, the color nicknames assigned to them have become difficult to untangle. Some are referred to by multiple colors, while others have changed entirely. A paper published in February by researchers at the University of Lisbon outlines how the science community is most commonly using the color designations today. It uses data from the last couple years to assess the price and economic impact of each color. Here’s a cheat sheet of where things stand.

Green hydrogen

Also known as “clean hydrogen,” green hydrogen is produced using renewable energy sources (particularly wind and solar power) via a process called water electrolysis. This approach doesn’t emit greenhouse gasses or carbon, making it the best for the environment.

At the moment, green hydrogen is the second-most expensive by a small margin, ranging between $2.28 and $7.39 per kilogram depending on available resources. (Used in a fuel cell to power an electric motor, one kilogram of hydrogen is roughly equivalent to a gallon of diesel.) It would need to be $1 per kilogram to achieve price parity with cheaper—and dirtier—hydrogen options. Only a tiny percentage of hydrogen produced today is green; in fact, all low-carbon types of hydrogen (that includes blue, pink, yellow, turquoise, and aqua) account for less than 1% of global totals.

Experts also note that there are a number of disadvantages to using green hydrogen for domestic energy, largely because it requires blending hydrogen with the gas that flows through your home’s pipes. Some say this could prolong the life of natural gas infrastructure and divert attention from renewable energy. Hydrogen is also highly flammable, meaning that above a certain concentration, there are safety concerns in domestic settings.

But while renewable energy is the ideal solution in homes, green hydrogen could still be promising elsewhere, like in long-distance transportation. The first-ever hydrogen-powered cargo vessel completed sea trials in the Netherlands in October and is set to enter service soon, and a plane run by a hydrogen-electric engine took flight earlier this year. Green hydrogen could also be used in construction settings, like steel plants, or for long-term storage of renewable energy. 

These applications are currently in early stages. However, according to Dr. Diogo Santos, an associate researcher at the University of Lisbon’s Higher Technical Institute and coauthor of the aforementioned research paper, there’s a very possible future in which green hydrogen becomes the leading hydrogen source.

Assuming that the global investment in renewable energy keeps increasing, Santos says there will come a point that the grid can’t absorb all the energy generated on a given day. That excess energy could then be used to create green hydrogen, drastically driving down its price and increasing its availability. Still, Santos and his coauthor predict that green hydrogen costs won’t be low enough to incentivize major government investment in large-scale renewable energy plants until the 2030s. 

Pink hydrogen

Pink hydrogen can also be called purple, red, or violet; they all describe hydrogen that’s produced using water electrolysis and powered by nuclear energy. While this technique doesn’t emit carbon, it can’t be considered renewable due to the resulting nuclear waste. However, pink hydrogen is generally cheaper than green hydrogen, ranging between $2.18 and $5.92 per kilogram. 

Santos believes that pink hydrogen could be an important solution—albeit a less sustainable one—for countries that are major producers of nuclear power, including France, China, and Russia. 

Yellow hydrogen

Yellow hydrogen is the third form that uses water electrolysis; yet here, the process is fueled by electricity from the energy grid. In the past, yellow hydrogen denoted hydrogen produced by solar power, but Santos says that categorization has since been subsumed by the broader green hydrogen umbrella. 

While yellow hydrogen is still relatively clean, it can vary widely based on the type of energy that makes up a particular region’s grid. In China, for example, the grid is mainly supplied by energy from burning coal, meaning that yellow hydrogen generated there would release considerable carbon emissions. By comparison, Iceland’s energy sources are clean, so yellow hydrogen made there would result in something close to zero emissions.

But even in countries with a lower capacity for renewables, yellow hydrogen could be a more sustainable option until it’s possible to make green hydrogen on a large scale since it’s typically cleaner than gray hydrogen. Still, it tends to be the most expensive of the water electrolysis options, landing between $6.06 and $8.81, depending on the region’s power mix. That cost is predicted to go down over time as energy production becomes cheaper.

Blue hydrogen

Blue hydrogen is considered relatively environmentally friendly, but the steps to creating it are a bit counterintuitive. It requires a process called steam methane reforming (SMR), which burns natural gas as its energy source. However, emissions are ultimately minimal because it catches excess carbon before it escapes into the atmosphere. One drawback is that some methane, a potent greenhouse gas, is released as a result. Carbon capture technology is also still in early stages of development and therefore is relatively inefficient, though it’s expected to improve with time.

Gray hydrogen, the most common type of hydrogen today, uses the same initial steps as blue hydrogen but without the carbon capture stage. This difference results in gray hydrogen emitting about five times as much carbon into the atmosphere. Blue hydrogen currently accounts for less than 1% of total hydrogen, but that could increase significantly if carbon capture systems are added to existing gray hydrogen facilities. Santos sees blue hydrogen as another possible low-carbon alternative until green hydrogen becomes more common. 

Turquoise hydrogen

Like blue hydrogen, the production of turquoise hydrogen is powered by natural gas. However, it can also be classified as a low-carbon option because the resulting byproduct is a solid form of carbon rather than a gas. Solid carbon can later be sold and used for industrial purposes like tire manufacturing. There aren’t currently any facilities making turquoise hydrogen, but several are under development. This method will require further testing before it can be used extensively. The price is expected to be considerably lower than green hydrogen—around $2 per kilogram.

Aqua hydrogen

In Canada, scientists have discovered a method of extracting natural hydrogen from oil sands and conventional oil fields that’s now termed aqua hydrogen. Santos says this process is interesting from a research perspective, but it’s unlikely to make a significant global impact and therefore shouldn’t be given the same weight as the other low-carbon options.

White hydrogen

Sometimes called gold hydrogen, this form is naturally occurring in continental crust; oceanic crust; or in volcanic gasses, geysers, and hydrothermal systems. There has been limited research on this form of hydrogen, and experts aren’t sure just how much of it there is. There also aren’t any current strategies to harvest this hydrogen. But because procuring white hydrogen would be carbon-free and require minimal infrastructure, Santos’ research posits that a hydrogen economy based on a combination of white and green might be the best answer for the transition to a carbon-free society.

Gray hydrogen

About 80% of all hydrogen currently produced is gray, which burns natural gas to power SMR. During SMR, carbon dioxide is released into the atmosphere. Environmentally, this method is quite bad, but it still only generates half of the emissions that burning coal for black or brown hydrogen does. Gray hydrogen is also the most cost-effective approach.

Brown and black hydrogen

Black and brown hydrogen are the least environmentally friendly; both are produced by burning coal. According to Santos’ report, this generates “as much CO2 as burning the source fuel would have in the first place,” or about 20 kilograms of carbon for every kilogram of black and brown hydrogen. This option is the second cheapest at $1.2 to $2 per kilogram. It’s also the second-most common, making up nearly 20% of global production. 

That means gray, brown, and black hydrogen—the worst forms for the planet—currently make up nearly 100% of the hydrogen market. In order to move away from these methods and toward low-carbon options, governments around the world need to make significant investments in renewable energy.