Photo from vreseis.com, image copyright Sally Fox
TIL you can grow cotton in colors besides white! As I understand it they can't be grown near white cotton for the usual specialized-variant reasons (they will cross-polinate and produce improper failcotton which is neither white nor nicely colored) which has allegedly kept them kind of obscure? But the colors are so nice!
The chemistry of it is particularly neat: they're not dyed, they're grown that way, so the properties of the cotton and how it handles are rather different than the colored cloths most of us are used to. This page (also vreseis.com) goes into a bit of detail about boiling in basic water (pH above 9) to develop the colors, or keep your washes pH-neutral to leave coloration unchanged.