Pish, I think the brown is simply toned billon. I don't think there's anything "under" there except unpatinated billion.
I have never tried "spot cleaning" at least on coins. This little sweetie cost GBP 8.50 shipped. Whilst it wouldn't be a financial tragedy if experimentation went belly up, I do feel a responsibility as the current custodian. I think it is fairly attractive as it is. If there is a chance that I might ruin it, I won't take the risk.
@TIF you are correct, it is toned billon. When I opened it under electric lighting, it looked like a freshly minted penny. In daylight it was toned down quite a bit. I had no qualms about sticking it in the soup to remove the green. It is not as orange as the sellers photos. It is a little patchy, but for the price I am not bothered.
Pish => I think your new coin looks nice I kinda liked the seller's sweet brown colour a bit better, but if the colour above represents its actual colour, then I like the photo above Ummm, I only have two Postumus examples ... This sweet galley example: ... and this nasty example (below) ... worst photo "ever" (wanna know my photography secrets?)
Here's mine. Nice dark silverish toning with hints of the impurities within the metal (no doubt this one is a billon, even if it does have plenty of silver). This is the seller's pictures, but in the hand the coin definitely has that darker grayish billon look of many coins from that era.
I have a RESTORED Postumus! Before: AFTER: RI Postumus 259-268 CE Antoninianus Cologne Providentia And one by his General Aureolus: RI Postumus struck by Aureolus 268 CE Revolt of Milan Concordia