Couleur (f.) = a substance to make metals flow more easily, a flux or liquifier.
We need to search all instances of “couleur” to make sure we have distinguished “color” and this substance.
See #123 "Check consistency of couleur": Mark as a material in all versions of the ms, and leave in French (and mark with Fr tag)
See also #110 and #123 for markup of "or couleur/e" and "couleur/e" respectively