Generally the Light-colored black walnut kernels have significantly milder flavor than the darker ones.
In case you prefer the light-colored kernels, hull the walnuts immediately after they drop from the tree, because allowing the hulls to partially decompose before hulling may cause a discoloration of the kernels.
The hulls are thicker and fleshy at maturity. They could be mashed and removed by hand, but of course, using some mechanical devices such as a corn sheller make the job much faster and way more convenient.
After hulling, the nuts should be washed thoroughly and spread out away from sunlight to dry for about 3 weeks. After that they should be stored in a cool and dry place.
Kernels that have been tempered before the shell is cracked are easier to remove. Soak the nuts in water for few hours, drain, then keep in a closed container for about ten more hours. The kernels should then absorb enough moisture to become tough, but still will remain loose in the shell.