For a natural tone... Browns, greens, blues. I think pale, foresty backgrounds would look nice. That would bring out the "nature" feel to me.
For basic forumy feel... Basic colrs. Reds, yellows, blues, maybe purples. Make liberal use of shading, I would like it
I have a suggestion though. There may be a nature like scheme, but if you ever have the time, I think and industrial Oddworld theme would be nice. Greys, pale blacks, browns, darker shades yet pale shades of red blue purple and green. City like backgrounds. I know that may be asking for some, but I know at least for me, I like to change color styles every so often, and darker colors are more calming to the mind.