Don't copy the .icns file in Finder. Double-click it to open it in Preview, then Command-A to select all and Command-C to copy. Now paste that into the Get Info window of the folder you want to change.
Don't copy the .icns file in Finder. Double-click it to open it in Preview, then Command-A to select all and Command-C to copy. Now paste that into the Get Info window of the folder you want to change.