Earth tone is a color scheme that draws from a color palette of browns, tans, warm grays, and greens. The colors in an earth tone scheme are muted and flat in an emulation of the natural colors found in dirt, moss, trees and rocks. Neutral and Natural tones are also considered "Earth tones".
From factmonster.com:
"Neutral (NOO-trul) colors don't usually show up on the color wheel. Neutral colors include black, white, gray, and sometimes brown and beige. They are sometimes called 'earth tones.'"
Many earth tones originate from clay earth pigments, such as umber, ochre, and sienna.
References
Earth tone Wikipedia(Text) CC BY-SA