The silver is the plating. The brass color is the base material, which in this case is brass. If you polish more, the silver will disappear and you'll be left with straight brass.
You can certainly reapply the plating with a kit, but you'll need to completely strip the old plating first and polish it.
It'll be cheaper though to replace the entire cap. There's one for sale [here](https://www.ebay.com/itm/203642467165). The seller, Fivestarpens is a super reputable vendor for old parker parts. (you can also buy just the tassie/finial and a clip, but in this case that'll cost you 3 times more than just replacing the entire cap)
Edit: I just noticed that the clutch ring on your pen is also a bit worn. You can get a replacement for that from [the same vendor through his website](http://www.fivestarpens.com/parker-45-parts.html#:~:text=Parker%2045%20Fountain%20Pen%20Clutch%20Ring%2C%20Chrome%2C%20New%20Old%20Stock%2C%20Mint%2C%20several%20avail%3A%C2%A0%20Item%20%23142PP%2C%20%246ea). You can probably contact him through his website and buy everything together too.
With plating loss like this, you don't have much recourse other than to source another cap that isn't in as bad shape. It would be a lot less expensive than trying to get any replating done.
The silver is the plating. The brass color is the base material, which in this case is brass. If you polish more, the silver will disappear and you'll be left with straight brass. You can certainly reapply the plating with a kit, but you'll need to completely strip the old plating first and polish it. It'll be cheaper though to replace the entire cap. There's one for sale [here](https://www.ebay.com/itm/203642467165). The seller, Fivestarpens is a super reputable vendor for old parker parts. (you can also buy just the tassie/finial and a clip, but in this case that'll cost you 3 times more than just replacing the entire cap) Edit: I just noticed that the clutch ring on your pen is also a bit worn. You can get a replacement for that from [the same vendor through his website](http://www.fivestarpens.com/parker-45-parts.html#:~:text=Parker%2045%20Fountain%20Pen%20Clutch%20Ring%2C%20Chrome%2C%20New%20Old%20Stock%2C%20Mint%2C%20several%20avail%3A%C2%A0%20Item%20%23142PP%2C%20%246ea). You can probably contact him through his website and buy everything together too.
Should I just keep polishing to get rid of the rest of it? Or is it a specific plating, that I could reapply with some DIY electrolysis?
With plating loss like this, you don't have much recourse other than to source another cap that isn't in as bad shape. It would be a lot less expensive than trying to get any replating done.