Blue, Grey, Green – oh my! The road to the emerald city of the future seems to feature a number of alternative energy projects with varying levels of promise. Perhaps the most touted yet least utilized at scale is hydrogen. But there seem to be many variations of hydrogen comprising a veritable rainbow of qualifiers.

What does each color mean and what is the practical application of each? And what lies at the end of this rainbow? We think through some of these here.



Hydrogen is the first and lightest element on the periodic table. While known simply as H, it is not naturally found alone, but in other molecules like water or methane. In the molecular form we use, it goes by H2. It is an incredibly useful gas because it is light but also combustible. Look no further than the Hindenburg for an example of each of these properties. But its uses are wide ranging and economically potent.

For energy, hydrogen is primarily used to power fuel cells or for direct combustion. The mass appeal and reason hydrogen gets so much attention is that it does not generate carbon emissions. Because there is only H in the chemical formula, burning it cannot produce carbon emissions, whereas burning natural gas, which contains CH4, will result in carbon on the other side of the equation. It is also transportable and can last in storage. While it is combustible – an attractive feature – it is also so light that it evaporates very quickly, limiting but not eliminating certain risks.

While hydrogen is not new, one issue is that it is not naturally occurring and must therefore be produced. The conventional ways of doing this utilize fossil fuels – or hydrocarbons. Right there in the name, one can already see the problem: carbon. The different color classifications, then, are general groupings to identify the carbon intensity of generating hydrogen gas.

For the most part, color-coding does not designate the method of hydrogen production as much as its carbon outcomes, so the same production process could be present in two color categories depending on other practices employed by the producer. In general, hydrogen is produced by steam methane reformation, coal gasification, or electrolysis.


Grey Hydrogen

Sometimes also called “black hydrogen,” grey hydrogen is relatively straightforward. Whether from methane or coal, intense heat is a catalyst in this process and that heat generally comes from fossil fuels as well. Because the heat comes from a combustion reaction and because the process separates carbon and hydrogen from their molecular bond, the process results in carbon emissions. Black and grey are used to tag these production methods as carbon emitting at the point of production.

Blue Hydrogen

The same process can turn from noir to blue simply by pairing a carbon capture or sequestration process. In this way, hydrogen is still produced from hydrocarbons through steam methane reforming or gasification, but the producer catches and stores some or most of the resulting carbon emitted.

If utilizing electrolysis, but the electricity is generated from a natural gas or coal-fired power plant, the resulting hydrogen would still be considered blue hydrogen. This means the color definitions reach back not just from the point of production, but in some cases to the previous step.

Green Hydrogen

To qualify as green hydrogen, the process must not be a carbon-intensive one. This usually means that the method is electrolysis rather than steam methane reforming or coal gasification, because electricity can be applied to water, which means no carbon is present. The power needed to generate the electricity, however is also important. Most definitions require the electricity used in electrolysis to be generated by renewables like wind and solar, but a more expansive definition would include nuclear, geothermal, and hydropower, as these also do not generate source emissions.

One trick to consider is that most color categories are imperfect because the entire economy is carbon-dependent going back far enough. The hydrogen may be considered green if wind farms supply electricity for the electrolysis, but of course there is carbon intensity inherent to the transmission lines, the construction of the wind turbines, the supply chains for materials, and the heavy machinery needed to mine for the raw materials. These are not generally contemplated in hydrogen production color schemes, which center around scope one and scope two emissions.

A horse of a different color?

There are other methods for producing hydrogen that do not fit as neatly into the color spectrum. Pyrolysis is one such method, which fits into multiple categories. Although reliant on fossil fuels, even without carbon capture and sequestration, pyrolysis is lower emitting than certain blue hydrogens, and when paired a carbon storage or use plan can be less carbon intensive even than green hydrogen. This has been labeled by some as turquoise hydrogen.

This unique process depends very much on technical specifics, but is worth investigating. Pyrolysis is the decomposition of methane from natural gas and does result in a carbon output, but rather than elusive and gaseous CO2, the process can generate solid carbon byproduct. It also uses a third of the electricity as electrolysis and does not generate emissions.

In the right context, methane pyrolysis can rely on existing infrastructure (e.g., natural gas productions and pipelines) without having to build costly and environmentally disruptive new infrastructure for transport and storage of hydrogen, nor require carbon-intensive mining for raw materials needed to produce renewables. Still other options like photolytic and biological processes may also work at different levels of scale, utilizing sunlight, algae, or bacteria to generate hydrogen.


The upshot of all of this is that there are diverse options for producing hydrogen – and if it is as valuable as experts say, they are all worth pursuing. Hydrogen can be used to advance vehicle electrification without the need for battery advancements because of fuel cells, and it can expand zero-carbon heat for commercial and industrial processes using the same natural gas pumped to those facilities. It is not primed for household service quite yet, so no natural gas stove tops will convert into hydrogen in the near future, but as we consider ways to produce this valuable resource, we should not forget the entire process needed to bring it about.

For many years, the color code has ensured we think about scope one emissions in the production of hydrogen, with some emphasis in recent years on scope two. The more we advance the conversation, the more we have to think about the entire value chain from the first point to the end use. Leveraging existing infrastructure to the greatest extent we can will save money and reduce the carbon intensity of the process.


Written by Benjamin Dierker, Director of Public Policy


The Alliance for Innovation and Infrastructure (Aii) is an independent, national research and educational organization. An innovative think tank, Aii explores the intersection of economics, law, and public policy in the areas of climate, damage prevention, energy, infrastructure, innovation, technology, and transportation.