most of them ARE neutral or deleterious
Don't emphasize the "are" when you don't actually know.
And the dark fur mutation is an excellent example of a deleterious mutation. It is clearly very deleterious and, being dominant, subject to strong selection pressure. That is why it is constantly stripped out of the populations.
That is, deleterious until there is a nearby dark lava field. Then suddenly it is extremely beneficial.
normally occurring dominant "D" alleles scattered through the population and not mutations.
and by what magic did one of them appear on one lava field and a different one on another but not both on the same field?