I've looked more into it. The 2 stories going around are that it's because of the one I've pointed out which has some substance to it, and the other more logical one is because DFB color is green.
Also, apparently the Swiss was the first country Germany played after WW2, but then again ... wouldn't that be like germany playing Austria? It's like cousins.
Anyways ... whatever the reason is, I like the one I've overheard better
It fits nicely with other jersey colour choice legends like Boca Juniors and others.