As someone who is starting to specialise in German stamps, I recently took the plunge and invested in a Michel Germany catalogue. I got the 2004/5 edition on
ebay for a very modest price indeed. I hope that very shortly I will be answering questions about German stamps here rather than asking them!
So, Australia Post willing, this is probably going to be my last call to Germany experts!
This is the November 1923 20 Miiliarden stamp. It was printed in brown on a pale green-blue background. However, I noticed the other day that I had a copy that was brown on blue-green. Here they are beside one another - making it clear that the two shades are very different:

Here is a larger scan of the green version:

I'm wondering if anyone knows whether this is a listed variety or perhaps, more excitingly, a case of a colour error.
My thanks to anyone who can be bothered looking this up for me!
BTW, a 1992 Michel catalogue is currently going for just $5 on
ebay. The prices may not be up to date, but even an older edition is extremely useful as a means of identifying your German stamps. (What's more, given the date this catalogue should have all the DDR stamps in it.)
Let me know if you'd like to contact the seller, who's in the US, and I'll tell you who he is.