Hello,
Gris argentin translates as "Silver Grey". By what I have seen, I would describe it as a light-medium grey with a slight blue tinge. Is is lighter that Gris-de-fer, that is "Iron Grey".
Regarding these peculiar colour names, I believe they were common at those times, and would not sound odd then, as they sound to us now. Some of them probably were very much theoretical and hard to distinguish, specially after some fading - take, for instance, orange, aurore and capucine, all three tonal variations of orange, and all three employed as facing colour by several Chasseurs à Cheval regiments.
BTW, if you have on hand the movie "The Duellists", D'Hubert, the character Keith Carradine interprets, is an officer in the 3ème Hussards.
HTH,
Dani