NPIP is a program to certify that chickens are free from particular diseases. In the state I live in, it's required to import chickens from another state, show your chickens or to sell them at a buy/sell/trade type of gathering.
As far as I know, no large hatchery sells red or colored Dorkings, just the silver gray. Sandhill does have several different varieties. It's a smaller family run business. I don't know what the quality of his Dorkings is like. It's quite possible that she is selling show quality chickens. A good silver gray is so beautiful. Reds are pretty hard to come by, as are the colored. I would love to have some nice reds! And some really good quality silver grays again! If they are nice looking chickens, I would pay that. They were my favorite breed, of all the chickens I've had.
Are you interested in getting some Dorkings? Breeding at all? If so, you could always talk to her a little bit and see if she shows them or how long she's been working with them.