Nice tutorial Burkhard
As you probably know 'mud' has been one of my pet trademarks over the years so here are a couple of other pointw to ponder which I have experimented with:
I usually follow the method you yourself use in the first part of the tutorial but there is one natrual flaw with it (the eye is generally kind enough to forgive it). :
The lighter shade should be on the bottom and not the top! - mud(the newest mud) is - and - mud is darker than dried mud!
On clothing of a lighter hue: tans etc use a dark shade only OR you a colour like red leather but the same method.
Now this IS sad, but I often when visiting battlefields, go loking for muddy puddles or bits of exposed ground to check out the colour of the mud!
Waterloo has in fact a very distinct shade of almost cream-white clay which leaves milky puddles all over the place on the battlefield on a - day.
Some soil in south africa is very red, so the mud is a distict copper colour.
Just two examples..
Hope this has not 'muddied' the tutorial