Red pants were worn by some zouaves. Their uniforms were based on the Algerians zouaves. The French originally recruited them and eventually had all French zouave units attired like their Algerian counterparts. Zouaves weren't necessarily trained in marksmanship (most Civil War soldiers weren't given marksmanship instruction).
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!