Hello Willi,
Since Watercolour is most often such a transparent paint in application and also since it is often built up through successive washes and or dry brushing as well. I am inclined to suggest you try whatever is approximately to your satisfaction with respect to whatever mix you come up with.
However you may try a variation of Yellow and Black as a base to work from although it is also valid using a Blue or Brown or a; .....
Colour mixes is in fact the easy part, the difficult part is in making the various washes give body to the whole while maintaining the colour you are intending to depict due to the transparency issue.
To further demonstrate the samples below were just made to answer your question. I could use any of them as a base for developing the Olive Brown streaked finish except for Olive Green without another colour or Hooker's Green as shown. Although with other colours mixed in both of those could also be used.
So the best option is to experiment to find what mud mix works best for you.
Cheers,
Daniel.