I'll wade in with my opinion... for what that's worth... ;p
Looking at the enlarged photos @ http://www.imfdb.org/wiki/Assassination_Games
the brass trigger guard is definitely, visibly a different color than the 'brass' colored frame, so I would have to agree that the lighting is to blame for the 'brass' coloring of the case-hardening... so no brassers.
Given the 'pin' front sight in the movie photo vs. the 'blade' front sight in the upper Walker photo, also given Mr. Van Damme is not a large-framed man... so hand to weapon size ratio in the photo (frame size/ weapon length) and loading lever catches on both weapons says they are Colt Style 1851's, NOT Walkers. Given the straight sided cylinder... they're .36 caliber.
Rounded barreled/Octagonal? Given the 'hard' lighting... as we saw in the post of the SC Museum weapon in another post here, it's hard to tell... till you peer closely at the barrel's end... then you notice full curvature.
If I had to take a guess... he's actually sporting a pair of Uberti 1862 Leech & Rigdons (irregardless what they may have called them)... given they almost match the photo (exactly) @
The only trouble I see with the movie photo, given no cap relief cut in the right side of the blast shield... is that the photo may be transposed? IE. he should be firing both weapons to the left of frame, instead of right.
Postscript: Searching yahoo photos of Mr. Van Damme, he wears his watch on his left wrist... so yes, the photo is backwards. Your looking at his left hand in the foreground... from the photo's right-hand perspective.