It is generally said that "earth tones" are browns, umber, terra cotta type colors. Perhaps "they" mean the color of raw soil as earth color? I think earth tones are all the colors we see in nature, which are mostly primary colors of red, yellow, blue and green.
My favorite earth colors are the ones found in fall, in Northern Ohio..The colors in my garden this fall are beautiful.
Even though it is the end of October, look what I found in the garden.