It was Jeff Ronne that dubbed the trait "pastel" quite a few years ago, and at first he believed it to be a recessive trait. After successive generations of selective breeding continued to become lighter and lighter, he concluded that it was in fact (as stated earlier) polygenic, and could be further refined with more selective breeding. The pastel trait itself doesn't make the beautiful pinks and oranges seen in some lines, but it reduces the darker pigments throughout the animal. This allows the colors to come through that much more.

-Evan