Walnut is often steamed to blend the differences in the whitish sap wood and the darker heart wood. Unfortunately the steaming does that well but tends to remove a lot of the beauty in the color of the Walnut. Steamed Walnut is kiln dried. Air dried or kiln dried Walnut that is not steamed will have much richer color, and obvious a stronger contrast between the sapwood and heart wood.
European Beech is often treated the same way too, for the same reasons, and also workability.
They can intermingled in the same project but getting a good color match would be difficult.
Dave