1A colour which something has by nature; specifically the colour of unbleached and undyed fabric or of unvarnished and unstained wood.
2In an image: a colour which accurately reproduces the actual colour of the object represented.
3A colour, especially of a fabric, reminiscent of the muted shades typically found in nature.
Late Middle English; earliest use found in Arderne's Treatises of Fistula in Ano. From natural + colour.