You are getting air dried lumber that stayed moist too long..if you want to use his lumber, but want it sharply contrasting, you likely need to buy it fresh cut. You can look at the contrast by water washing the board at that point. And if it is the contrast level you want, take it home and dry it yourself. He may have had a couple logs that laid out on the yard for a time. If that is the case, the color was in there when it was sawn and you merely need to leave it with him and buy a different board.
As mentioned by several others, once dry, hand plane. Harbor fright sells a very small one that you could put in your pocket when you visit your supplier.
I have sawn both type of trees, prestined and clean high contrast. If your Sawyer cuts a lot, he won't know which board is from which tree. If only a few, he likely can.tell you which is best contrast boards.
Honestly, very few keep the sapwood. It is viewed as a defect in commercial furniture which is why the steam treatment to pull more color into it.
