Journal of Renewable and New Energy

Hydrogen Rainbow

Document Type : Review Article

Authors

Sara Abdi 1
Masoud Nasiri 2

1 Assistant Professor, Faculty of Chemical, Petroleum and Gas Engineering, Semnan University, Semnan 35131-19111, Iran

2 Associate Professor, Faculty of Chemical, Petroleum and Gas Engineering, Semnan University, Semnan 35131-19111, Iran

10.22034/jrenew.2026.247781
Abstract
Hydrogen can be used as an alternative energy carrier to achieve energy systems with zero or near-zero carbon emissions, if it is produced from renewable energy sources or through low-carbon processes. Hydrogen is a colorless gas that is assigned symbolic colors to classify it based on the resources used in the production processes and energy sources. Methods such as water electrolysis, biomass gasification, and steam methane reforming with carbon capture and storage provide pathways for hydrogen synthesis. This study examines the main pathways associated with hydrogen production and the colors assigned to each pathway, known as the “hydrogen rainbow”. Based on the hydrogen source, production process, and carbon dioxide emission rate, hydrogen colors of green, gray, brown, black, blue, turquoise, orange, pink, red, and yellow are explained in detail in this review. The color classification of hydrogen production based on greenhouse gas emissions provides a framework for understanding their environmental and economic characteristics. The environmental benefits of using hydrogen depend on the hydrogen production methods and the primary sources used. Only green hydrogen has very low greenhouse gas emissions. Blue hydrogen is also produced by incorporating carbon capture, utilization and storage technologies based on gray hydrogen processes, to significantly reduce its carbon emission intensity compared to gray hydrogen. On the other hand, the introduction of an international market for hydrogen is important to reduce costs and produce hydrogen under the best conditions.

Keywords

Renewable energy
Hydrogen production
Hydrogen colors
Carbon emissions
Low-carbon hydrogen
Subjects
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  • Receive Date 16 August 2025
  • Revise Date 24 February 2026
  • Accept Date 23 June 2026

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