The brass 'color' on the nickel bolster is probably brass swarf (dust) embedded onto the nickel of the bolster from the hafting process. The 'hafting' is when they shape and polish/buff the bolsters & handle after the knife is put together, on powered grinders & buffer wheels. The brass swarf taken from the liners gets transferred via the belt/buff onto the surface of the nickel. Brass dust taken off in powered grinding is messy, and it can stick/cling to everything, especially under the forces imparted by powered finishing tools. It's color makes it really stand out.
You could probably clean that up with some high-grit wet/dry sandpaper, something approximating the finish from the factory. Something around 600-800 grit should do it. It might come off with some metal polish; but if not, then the high-grit sanding should definitely clean it up.