Exercise: take a white piece of paper, cut a window into it, pin this to a tree; paint the tree bark visible through the window. Carlos Villa introduced me to this learning tool.
This is especially good to do with watercolors. As you observe and then attempt to replicate the colors of the bark (oh, just simple gray) you’ll keep finding more and more embedded color. The process is almost hallucinogenic. Each time you think you’ve found the match you see another color in the window.
To a casual observer, tree bark is gray or brown or maybe a reddish color. To someone who has tried to paint it the bark is many colors.