Thanks!  It's not hard, if you know the trick - which is to start with a medium color first, fill the area flat.  Not the darkest and not the brightest, but intermediate.  The trick works no matter what color this is - you can start even with blue grey, IF THE LIGHT IS BLUE.  That's important.  The flat medium color must be chosen with respect to the color of the light hitting it.  If it's neutral pick brown.  

Then darken it, to paint shadows, and - here's the trick - make it warmer.  Add reddish-orange to it, just a bit.  
And of course do the opposite for the lighter parts - reduce saturation of the brown, or if it's already grey add a tiny touch of cyan (sky-blue).   That's it.  :)