Heres a Illustration I painted for a friend, some years ago, I think in 2008.
Scruffy the Tree Cat, 11X17, Oil on Gessoed Board.
I painted all the tree bark with mixtures of Titanium White, Cad. Yellow Medium, Cad. Red Light, and Ivory Black. Its been a while, but I'm pretty sure my Mediums for this were Gamsol, and Liquin.
There is a better detail pic on my blog: