Certainly some have a tinge of brown to them, and pines look almost Army green in spots, but from a distance tree trunks appear gray. So why do we simplify them to brown when we draw them? Are there trees in another part of the country that truly do have brown trunks?
Last winter I took an introductory watercolor course and the instructor, Joyce McJilton Dwyer, said that the trick to realistic art is painting what you actually see, not what you expect to see. I bet she's known about the tree bark issue for ages.