I've had this happen to me. The seller should do whatever you agree to. When it happened to me the seller sent me a new snake and I paid for shipping. I kept and sold the first snake I was sent.
In my opinion there are a few options that don't involve the seller paying for shipping....
1. Give you a discount on a new snake equal to the price you paid
2. Send you the correct snake (you only pay shipping)
3. Give you at least half your money back, all of it if you don't like the snake/want it
(my least favorite option unless it's a full refund minus the shipping)
IMO you paid for the snake you saw in the pictures. You didn't email the seller and ask for any old pastel, or whatever you are looking for. If you wanted to do that you would order from some crappy pet store website.