Hello, welcome...
What's "gust discolouration"?
I guess if the parts are steel, rust might be possible. But I would think they would be chrome-plated plastic? As long as the chrome is intact (not flaking/cracked), I would expect that you can clean it with steel wool (the pads that have green soap in them) and polish with Autosol (a German metal polish in a gold/black tube) if necessary.
If it is the regular shiny type of chrome, it is likely to be quite hard and will stand such treatment, but if it is peeling off, that's a different story.
If the parts are 'brushed chrome', then it is probably going to be very difficult not to ruin their finish. They are an aluminium paint-type finish and it would be best to ask a paint shop for refinishing advice.
New parts will be available from the dealer at great expense (the bar on the tailgate, with the grille, is at least £200 - I investigated because I was thinking of putting it on a standard 500).
-Alex