Feed has no effect on the color of the shell whatsoever. The more natural the feed (ie: bugs, lizards, plants, etc) the deeper the color of the yolk. The color of the shell depends entirely on the breed of hen.
I raise Araucanas who lay shades of blue, green, pink, brown, and, occasionally, white eggs. I, also, raise Buff Orpingtons who lay various shades of brown - from very light, almost white, to deep, rich brown.
A way to tell if the egg is fresh, once you break it, see how high the white (yes, the white) of the egg stands up. It should not run much, except near the edges. The whites of my eggs stand higher than the yolks of store-bought eggs.