Austria 116

Is this a common error in the color??

I thought the same but notice that the stamp on the left has fading inconsistent. It's not like the stamp was left out uncovered in different locations. I also just received this old collection and none of the other stamps were like this. They were in a homemade album and the last entry was 1922. You can tell the pages are very old.

I do like that series!
But I don't know of any colour variations. They look to me to have been exposed to sunlight for an extended period of time. Notice how you can see, especially on the left hand stamp a straight edge of colour change over on the right side, as if the edge of something was laying there covering part of the stamp.
These stamps are scarlet in colour. I've often noticed that the red pigments fade first on exposed printed material, like posters.
That would be my guess, anyway.
And...welcome to the forum, Tazzman!

There are two distinct varieties of this stamp. One is on coated paper and one on uncoated paper and the colours are noticeably different. The coated variety is red and the uncoated is orange red. The same varieties exist for all values in the set up to the 35 heller. You will probably also notice minor shade varieties as well.

Thank you. This is probably just fading then.

I think you have the two varieties.