Re: pine bark brown
I assume you already have Ultramarine Blue (it being a "basic color"). Try some burnt sienna, or burnt umber, with just a touch of Ultramarine Blue, and a little white. A wide variety of browns can be made that way.
When I think of pine bark, I think of a rather grayish brown (or, brownish gray). The afore-mentioned colors can get you there. If you happen to have Raw Umber, also try that, again with a touch of UB and white.
But ultimately, you will need to paint different colors for different parts, depending on lighting and surroundings, as was already noted. So, don't get obsessed with finding the perfect color.