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I'm not a metallurgist,but the rough idea is about higher levels of carbon in the iron/steel.
The low carbon parts were packed in leather chips and/or bone meal in an iron box..Various salts ,cyanide,etc might be part of a proprietary recipe.
The loaded box would be baked at roughly red heat for a prescribed time,then quenched in agitated water.
Its sort of like smoking the bark into a beef brisket.
The carbon would cook into the surface of the workpiece,increasing the capacity to harden in quench.
The colors were a by product and they gave a visual indicator of quality.
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