In The Outlaw Josey Wales, Capt. "Redlegs" Terrell is seen shooting an 1851 Navy that clearly has a brass frame. In the movie Glory, Matthew Broderick, in the assualt on Fort Wagner removes his Navy Colt and blows the sand out of the cylinder. Now the frame on his Navy is rather shiny. At first I thought it was nickel plated, but now I'm not so certain.
Seems to me that this might be Hollyweird's doing.