didn't know dark hair faded

i just tried to go back to my natural dark brown hair - i used preference medium golden brown and it looked really good - i loved it but to my surprise 2 weeks and 3 washes later (my hair is almost always up in a ponytail and doesn't get oily so I can get away with not washing it often) the color has almost totally faded (and this is with really no sun or no pool)
what is up with this
if i'm going to have to do this every month i may as well go with like natural instincts, right?
any thoughts? (ie is this amount of fading normal, and should i go with natural instincts, is this fading just b/c preference is not a quality product)
thanks for any input