There’s one energy source the world is increasingly betting on in the race to decarbonize the economy: hydrogen.

The most abundant element in the universe could be a game changer in reducing emissions from steel, cement, ships and more.

While general agreement exists on that potential, disagreement is emerging on everything else: how to ensure it’s clean, how to prioritize its use and what policies are best to kickstart the industry.

The hydrogen debate has been…colorful. Grey, blue, green, pink and other shades have become a common parlance among energy experts in discussing hydrogen’s emissions profile.

Using colors to label hydrogen is an approachable way to debate the future of a nascent industry—but it’s also rigid, oversimplified and doesn’t capture the whole picture.

“There is a whole system involved with making and using hydrogen, and the whole thing needs to be accounted for in assessing carbon intensity,” said Adria Wilson, a policy specialist at Breakthrough Energy with expertise on hydrogen. “We need to be doing lifecycle analyses instead of focusing on the point of production.”

Let’s first explain what the colors stand for then get to why they can fall short. The following is nearly all of them, but new ones regularly crop up in the debate.

Grey hydrogen refers to a process called steam methane reformation, which makes hydrogen from natural gas or coal gasification. It’s a dirty option because CO2 is emitted. Nearly all of today’s hydrogen supply is made this way and it mostly serves as an industrial feedstock.

Blue hydrogen is produced the same except the CO2 is captured and stored (how much is captured matters, as we explain below). This represents just 1% of hydrogen production today.

Turquoise hydrogen is produced through a process called methane pyrolysis, which generates solid carbon. It’s cleaner than grey hydrogen but dirtier than green. Production is in the experimental phase.

Green hydrogen refers to hydrogen produced with renewable electricity through a process called electrolysis, which splits water into hydrogen and oxygen.

Green hydrogen, which barely exists today, is what’s getting most of the attention lately. Gas-exporting countries like Norway and oil and gas producers support blue hydrogen, too. (See the below Data Dive for more).

Pink hydrogen follows the same path as green, except the process is powered with nuclear energy electricity, which doesn’t emit any emissions but does generate nuclear waste.

These last two types, together with another option that uses biomass through gasification, all result in near-zero greenhouse gas emissions, according to a 2021 report from the Hydrogen Council.

These colors represent the energy sources by which hydrogen is made, but a lot more goes into making hydrogen usable in our economy. That’s how things get complicated.

Take transportation.

The European Union wants to import half of its 2030 estimated share of renewable hydrogen use. Germany, for example, is looking to import renewable hydrogen from Australia. This raises questions about how to consider the emissions arising from transporting the hydrogen on a long journey, including the cleanliness of maritime fuel.

Liquefying hydrogen, needed for transport by sea, is three times more energy-intensive than liquefying natural gas, according to CleanTechnica. Hydrogen could be transported as ammonia but reconverting it back would require additional energy.

Water access challenges can arise. Countries across Africa, for example, have plenty of sun but are surrounded by desert and don’t have abundant water access. Renewable hydrogen production in those areas would require desalinizing ocean and seawater, which is also set to drive up energy use.

Let’s turn to blue hydrogen. Supporters say that capturing and storing the carbon from the current dirty ways we produce hydrogen can be a good solution in the short term to reduce emissions. Critics say it’s a license for oil and gas companies to keep polluting.

How much carbon a hydrogen plant captures makes a big difference. For example, one plant could capture 45% of the carbon while another could capture 90%—and both be seen as “blue.” The EU is set to develop rules to better define this type of production.

Even a hydrogen plant that captures carbon at a 90% rate does not reduce emissions by 90% because additional energy is needed to power the capture and storage process.

Methane leaks also occur during the production of the natural gas used to make the hydrogen, though how much actually leaks differs depending on technologies used.

Concerns are also arising about whether the hydrogen itself escapes into the air. Depending on how it’s made, distributed and used, it could make climate change worse, according to research under peer review now by the Environmental Defense Fund, an environmental nonprofit group. The research highlights the need to set up the hydrogen economy correctly from the start to prevent leaks, EDF said.

The best way to assess hydrogen’s carbon footprint is by spending a few more words being more specific about the entire process and meaningful differences in emissions, said Galen Hiltbrand, senior analyst at consultancy Rhodium Group.

Instead of saying blue hydrogen, we could refer to hydrogen from a plant with 90% carbon capture rate; or instead of green hydrogen, we could say hydrogen from electrolysis powered by offshore wind.

Editor’s note: Breakthrough Energy supports Cipher.