Well, maybe the images aren't good enough, but it's distinctly more copper red than the usual yellowy brass clad ones. I found it in a lot with maybe fifty others, both "Bank deutscher Länder" and "Bundesrepublik Deutschland", and the rest of them are all yellow. But maybe it's just patinated differently.
The high points and worn areas (rim and 10) are indeed the color (yellow brass) you would expect. The rest is simply caused by whatever environment the coin endured and appears as just a type of patina in my opinion.
I agree that different environments can cause different colors of patination. Brass is a mixture of mostly copper plus some zinc. In some environments, including high chloride solutions, more (or all) of the zinc is removed from the brass at its surface, yielding an oxidized copper color. This is called dezincification of brass in corrosion studies, and can affect everything from plumbing to Navy ship parts. It obviously starts at the surface, as in the case of this coin, but with very long exposure can eventually penetrate the entire object and it will then crumble into dust.
Thank you, CobaltBlue, this is a possible explanation, although in some parts it looks like actual, non-oxidized copper. In any case I think I'll keep it, it's actually nicer like this with its red copper sheen.
