The only French mustard that has a legal certification (an "IGP", meaning "Certified Geographic Origin") is the "Moutarde de Bourgogne". It's made with wine while the Dijon mustard uses vinegar. And the latter (Dijon's recipe) imports most of the mustard seeds used in its mustard, while the "Moutarde de Bourgogne" has to use locally produced seeds.
https://fr.wikipedia.org/wiki/Moutarde_de_Bourgogne
By the way, mustard was a very important condiment in France for many centuries. But out of the 8 French idioms listed in a 150 years old dictionary, only one is still used nowadays.
https://www.littre.org/definition/moutarde
"Moutarde de Meaux" (or "Moutarde à l'ancienne"), a mustard done the old way. One of the best producer is Pommery:
https://www.moutarde-de-meaux.com/pommery
"Moutarde de Normandie", the same than the above but with cider vinegar:
https://toustain-barville.com/3-moutardes
"Moutarde violette de Brive", yep! it's purple:
https://www.denoix.com/moutarde-violette
"Moutarde à la Violette", this one is not purple, but do include extract of violet flower:
https://www.lamaisondelaviolette.com/fr/epicerie-salee/101-m...