By maritime and logistics professional Mark Cooper

Like many of us in the shipping industry you may have noticed increasing discussion around hydrogen and its potential to displace traditional fossil fuels which contribute heavily to global greenhouse gas emissions. The International Maritime Organization [1, 2] reports that the shipping industry contributed 1,056 million tonnes of carbon dioxide or around 3% of global greenhouse gas emissions in 2018; this was a 9.6% increase since 2012. Further, the study found that general container ships, oil and gas tankers, and bulk carriers combined contributed 86.5% of total international shipping emissions.[3] In line with the United Nations Sustainable Development Goals, the IMO has mandated reduced emissions for new ships produced from 2022 by between 30% and 50% depending on deadweight tonnage.[4]

Given this drive to decarbonise, many are looking to hydrogen and hydrogen derivates (such as ammonia and methanol) as an alternative to fossil fuels.

Hydrogen (H) is an interesting atom. In nature it exists in a stable double bond (H2) configuration and is the smallest molecule in the universe. When burnt, the flame is colourless so why do we talk about different colours or hues of hydrogen?

Before we discuss hydrogen colours further, it is important to understand that hydrogen it is not an energy source, but rather a carrier of energy. To illustrate this principle, consider a process where the electricity produced via sunlight (the energy source) on solar panels powers a system which uses electrolysis to split pure water (H2O) into hydrogen and oxygen. The sun is the source of the power, but the collected hydrogen is the energy carrier. The energy carrier (the hydrogen in this case) stores the energy until it is burned to release the stored energy.

Regardless of how H2 is produced, all molecules of H2 are identical in every aspect. If that’s the case, then what is green, blue, grey, and even pink hydrogen? Well, remember that we need a source of energy to produce H2. In this equation, the energy source used to make the hydrogen is the determinant of the hydrogen colour.

It is also worth noting that the ‘colours’ described below are not formalised so in some instances may indicate a different source to different audiences.

Green hydrogen is made using energy sources that are renewable and emit nil carbon dioxide in the process. Examples are solar, wind and wave swell derived electricity used to power an electrolyser which uses electrochemical reactions to split water molecules into hydrogen and oxygen. Green hydrogen is currently expensive to produce but cheap green hydrogen is the gold standard many in the industry are striving for.

While green hydrogen can be made using electricity from solar power, the descriptor yellow hydrogen is sometimes (perhaps incorrectly) [5, 6] attributed to hydrogen made solely from this power source. More commonly, yellow hydrogen is said to derive from energy with a mix of renewable and fossil-based sources, e.g., wind and methane or solar and diesel. [7, 8]

If hydrogen is found naturally, such as in underground deposits, it is termed white hydrogen.

In its simplest form, blue hydrogen is typically made using a process (typically steam methane reforming) where natural gas (or another hydrocarbon source) and steam are introduced to a vessel and reacted in the presence of a catalyst under high pressure and temperature.[9] The resulting product is commonly called syngas or synthetic gas. This syngas can be further refined to remove remaining contaminants if pure hydrogen is required. Crucially, in this process, waste carbon dioxide and carbon monoxide are captured and stored rather than being released to the atmosphere. Blue hydrogen does not contribute to greenhouse gasses but does use non-renewable energy sources to power the production processes.

Grey hydrogen is the same as blue hydrogen except the carbon dioxide and carbon monoxide are released to the atmosphere and contribute to global warming.

Turquoise hydrogen is produced using a methane pyrolysis process. The resulting products are hydrogen gas and solid carbon products. The benefit of this method is that the solid carbon is stored rather than released to the atmosphere. For the technical minded, pyrolysis is the thermal decomposition of methane (or other materials) in the absence of oxygen.

Black hydrogen is made using black coal or through the gasification of certain other hydrocarbon sources. Likewise, brown hydrogen is made using brown coal (lignite) or through the gasification of certain other hydrocarbon sources. Both forms are damaging to the environment.

When nuclear power is used to generate hydrogen via electrolysis, the result is pink [10] or purple [11] hydrogen but some pundits [5] also term it as red hydrogen. Pink and purple are the more commonly accepted colours for nuclear power derived hydrogen.

When hydrogen is made with electricity that is generated from biogas or biogenic waste, it is termed orange hydrogen.[12, 13] Biogas is formed when organic matter decomposes in the absence of oxygen to form primarily methane and carbon dioxide. Typical biogas sources are sewerage, landfills and decomposing agricultural waste.

Red hydrogen is hydrogen derived from the gasification of biomass. [14]


1. International Maritime Organization, Fourth IMO GHG Study 2020. 2020, International Maritime Organization.

2. Organization, I.M. Fourth Greenhouse Gas Study 2020. 2020; Available from:

3. International Maritime Organization, Fourth IMO GHG Study 2020 Executive Summary. 2020: London.

4. International Maritime Organisation. Initial IMO GHG Strategy. 2021; Available from:

5. nationalgrid Group PLC. The hydrogen colour spectrum. 2021; Available from:

6. Haynes, A. The difference between green hydrogen and blue hydrogen. 2020; Available from:

7. Giovannini, S. 50 shades of (grey and blue and green) hydrogen. 2020; Available from:

8. Dodgshun, J. Hydrogen: Clearing up the Colours. 2020; Available from:

9. American Institute of Chemical Engineers, Hydrogen and Syngas Production and Purification Technologies. 2010, United States: Wiley.

10. Gas Connect Austria. The colours of hydrogen. 2020; Available from:

11. Taylor, K. Hydrogen produced from nuclear will be considered ‘low-carbon’, EU official says. 2020; Available from:

12. Appunn, K. Government defines “green hydrogen”, decides rules for rapid market ramp-up. 2021; Available from:

13. Haas, G., F. Alexander Wesche, and F. Tepper-Sawicki. An opportunity for biogenic hydrogen? 2021; Available from:

14. H2 Industries. Hydrogen – in Colors. 2021; Available from: