Hydrogen can be produced or extracted from many different sources. Because the production method is the primary driver of lifecycle emissions, hydrogen from different sources are often referred to as different "colors" of hydrogen. The list of colors chosen here is nonexhaustive and somewhat subjective/arbitrary but reflects my professional understanding of the general industry consensus on hydrogen colors. There are a near-infinite number of ways to make hydrogen and very few have widely agreed-upon "colors"; if you feel I missed your favorite color, drop an explanation in the comments.

"Hydrogen cost" should be understood to reflect "average national market price in the US, including relevant subsidies and tax credits".

For some context, current hydrogen prices (at the time of writing and with substantial error bars) are ~$1/kg (black/brown), ~$2/kg (gray), ~$4/kg (blue), ~$6-8/kg (green), with other colors not produced in commercially meaningful quantities.