Portugal - A Stamp "Negative"

Probably just usual, but I am unable to Google anything out about them... maybe I am using a wrong key word :)

It looks that these stamps are printed in a "negative" form... is that an error, or it had some purpose?

The stamps look to be, left 1884 SC59 with a 1892 provisorio overprint and, right 1892-93 SC72. However there are many later reprints.
There's a note in Scott's about a brownish gum which stains the paper on some of these reprints. There is a rather lengthy explanation of the early Portuguese stamps and their reprints at the beginning of the Portugal section. Too long and complicated to relate here. It also includes differences between original prints and reprints. Phew!
I think there's some obvious staining here, whether because of gum or what ever, that gives the negative appearance.

What you have is sulphide blackening of a possibly chalk
surfaced paper and a pigment that contained white lead.
Reasobnably common on the Portuguese
Can be repaired

Here is one, where half has been restored

Wow what a difference rod.
On the stamps you show.
How do you repair a stamp like that. And finally can it be harmful for the stamp to do so.

I am a little hesitant to offer any advice that involves
meddling with stamps, perhaps read this SCF discussion?

https://goscf.com/t/6683

Thanks James and Rod!

I just made a "negative" in my photo editing program, and I get a much nice photo of the stamp...



that was a reason why I thought that it is some kind of unusual printing.