First thing that comes to mind is powdered charcoal. Not sure if it would achieve the desired tone your looking for but certainly an inexpensive and local source to look into.
If nothing else, it could be added to reduce the amount of a commercial colourant needed.
I've never used coloring for an earthen floor, but I've used this company's pigments for adding color to lime plaster. They have a range of different products from more raw, unprocessed ochres to oxides with "higher tinting strength and opacity."
I had been wondering about wood ash in a cob construction, to maybe give it a darker color. Wasn't sure though how it might react chemically. I guess maybe this is the bio-char that the previous poster mentioned. I wasn't aware that it was a thing before now. I have heard others mention adding different mineral dusts to earthen constructions as well, for color. Not to sure on the specific details though. I've also read about adding ox blood to cob. I think that had a different purpose than coloration, but it probly would give it a very dark brown color.
My builder used iron oxide and black tea to make a reddish earthen floor; it looked beautiful, like shiny terra cotta tiles the day I moved in. But it turns out it crumbled too much so now it's a bit more like a beach. And it turns all my socks red.
Works at a residential alternative high school in the Himalayas SECMOL.org . "Back home" is Cape Cod, E Coast USA.
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