They are either found in the wild or they pop up randomly in people's collections. They are not "created" by people - they are discovered and then proven.
The Pastel story (from Kevin McCurley's book, The Complete Ball Python: A Comprehensive Guide to Care, Breeding, and Genetic Mutations):
The original Pastel was an imported male that I bred to a normal female, and hatched normal-appearing babies as a result. At the time, ball python breeders had little experience with co-dominant traits and it was assumed that these normal offspring were heterozygous for a simple-recessive gene. I raised the offspring and bred a female back to the adult male, subsequently hatching out three visible Pastel Jungles. During that time, another breeder (Greg Graziani) also hatched out Pastels to confirm that it was in fact a first generation morph.