'Brun' got used occasionally, but I would say 'chatain' was more common. 'Rousse' was used once, in reference to the beard of a black-haired man. In modern English I would say that chestnut hair is brown with a reddish tint: a chestnut horse, however, is most definitely ginger.
Well, I was mistaken in citing the possibility that Docteur Querelle had provided the details of Cadoudal's appearance. From the very extensive Appendices titled 'Piéces Justificatives ' in Georges Cadoudal et la Chouannerie (1887), here is the SIGNALEMENT circulated by the Prefecture de Police in September 1802 when Georges was still in England:
PREFECTURE DE POLICE
(Ier Division. No 9454.)
Paris, le 28 fructidor an X de la République
francaise, une et indivisible.
Signalement de Georges :
5 pieds 4 pouces.
Tête un peu grosse. .
Joues rondes et pleines.
Nez mince et relevé, yeux gris, petits, un peu enfoncés.
Cuisses et jambes tres-grosses, ainsi que tout le corps.
Cheveux presque roux, épais, frise a la Titus.
Peu de favoris, un peu rouges.
(Archives de M. Gudstave Bord.) "
(There seems to be a certain emphasis here on the redness of hair and side whiskers (favoris- I had to look that up) )
Those details had evidently come to light since 16 Fructidor, when the office of Conseiler d'état de prefet de police had circulated instructions "de rechercher partout avec le plus grand soin les individus dont voici les noms et les signalements" for three Royalists, Limolan (sic), Hyde, surnomé Neuville, & Georges in which for the last they were only able to say "ex-chef de chouans"
(Appendix No. 83)
For those who are curious, Appendix 88 contains further, increasingly detailed descriptions from February and March 1804 (no Fotofit in those days).