Natural is the most common colour, so it is really not worth anything at all.
Sepia isn't worth that much either, but it's worth somewhat.
Same with cream and black, but a bit more.
Silver an blonde could get you a few mil.
Calico and Achro could get you a really good amount.
So can albino. :3
But for Seasonal, CS or Uldavi pets that's a whole different story.
They are worth more, even if they are nattie.