Beyond the Rainbow: Aligning Hydrogen Color Codes with CO₂Emissions for a Transparent Energy Transition

Hydrogen is promoted as an essential building block in the energy transition, yet color codes used to describe its production pathways lack consistency.Color labels (e.g., green, blue, grey, black) are adopted in policy and industry discourse but fail to reflect carbon dioxide emissions.This article examines color classifications, building on an existing framework by expanding it through a qualitative assessment of recent literature.It analyzes how various production technologiesincluding water electrolysis powered by renewables, steam methane reforming, coal gasification, and nuclear-based processes -are labeled.Findings reveal significant inconsistencies, particularly for fossil-based and nuclear hydrogen, and highlight that renewable-based hydrogen is uniformly classified as green regardless of production specifics.The analysis suggests that if color codes were reassigned based on actual emissions, fossil-derived hydrogen would fall into a narrower, darker spectrum.These results call into question the compatibility of non-green hydrogen with climate objectives of the Paris Agreement.