We may both be incorrect here and someone could straighten our thoughts, but as far as I am aware, heterozygous means that a offspring has received 2 different copies of a certain gene at one location, one from each parent.
So say Piebalds gene location was named “P”.
The “normal”/WT mutation = p (lower case)
The Piebald mutation = P (upper case)
A normal would have p-p (normal looking)
A het would be P-p (normal looking)
A visual would be P-P (piebald)
For the mutation to take place, in a recessive, both need to be “P” rather than “p”.
The reason a “het” doesn’t show through is because the WT gene “p” is dominant, where as the mutation “P” is recessive.