This is the confusing thing with working with common names and not scientific names. If we worked entirely with scientific names, you would see that tri colors, Lystrophis semicintus, are not the same as "true" hognoses, Heterodon nasicus (for example). True hognoses are of the genus Heterodon, while the common name has been extended to Lystrophis of South America and Leioheterodon of Madagascar, because of superficial traits (like the nose) and other similarities in natural history. I am not sure of their relatedness, so someone should post on that, but I can assure you that they are still distantly related.
Therefore, no special breeding has been done to a true hognose to get the special colors, tri colors are that way because they are their own species!
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"I know the human being and fish can coexist peacefully."
Governor George W. Bush, Jr.
"Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us."
Calvin and Hobbes (Scientific Progress Goes 'Boink', 1991)