I think the brass and copper finish are electroplated just like chrome, but they do oxide and get dark, so they use this varnish to protect it. The chrome is more robust as it doesn't need the protection.
I have a small spot at my MCAL boiler (brass finish) as the plastic container I used to use have scratched the finish. It's very delicate indeed, so +1 to water and cloth. The chrome will take longer to be removed with cleaners (polishers for instance) but it will too...

The chrome at the welded parts seems to yellow first and I've noted the same on a 73' La Pavoni.
Just to add some info on the base - it's steel, and it suffers a lot with coffee spills that stays between the Elektra sticker, plastic base and buttons. This Semiauto that I've got had the base with several rust around these parts - and it's hard to see and clean. Even if one wants to do the chrome (or copper) finish again, the rusted parts are hard to polish to get a nice finish.
So, I chrome it again and it's better. I removed the sticker rivets and replace them with a M3 screw (rounded allen head) and lock nut. I'll do the same to the MCAL as I've seen some coffee between the sticker and base and it's impossible to clean it!
Márcio.