Ever heard of green cotton? I hadn't either, until recently. It's a natural color but fell out of favor when white cotton became easy to grow. White cotton can be dyed all sorts of bright colors so people preferred to use that instead of the colored yarn. (Turns out, cotton comes in all sorts of muted colors - green and brown are the two I have recently become familiar with.) The photo below shows a dishcloth I knitted using brown cotton along with dyed white cotton. 

Hobbii Yarn - nice price, pretty yarn but, alas, a little thin and splitty. But pretty!

I have used it double with a solid color for thicker socks and that worked out well.