Why is the skin of European bog bodies brown?
The anaerobic decomposition of moss produces tannins in the peat.


  The bog water interacts with the acids in the moss and produce tannin
  and other chemicals that preserve the bodies that fall or are cast
  into the bog; hence the numerous well-preserved bog bodies.
  http://www.digitalmedievalist.com/opinionated-celtic-faqs/bog-bodies/


The tannins tan the bodies just like tanning leather; the color of the bodies darken and the tanning action also preserves them.