The greenish is an intergrade between the black rat snake and the yellow rat snake where their ranges overlap. A cross is a man made animal. For those purists, locale intergrades are the only "true" intergrade. For instance, if you took a pair of intergrade greenish rats from the same locale and bred them, you would get a true locale intergrade. Now if you took a very nice looking yellow and a nice black rat and bred them for the greenish trait, you would get a cross greenish, not a true intergrade, but a greenish nontheless, just not a "true" intergrade. Most of the man made crosses are more colorful than the natural intergrades. Not all, but most are. There are some intergrades out there that just cannot be improved upon.
Just like here in KY, we have the intergrade red X eastern milk. They are a naturally occuring intergrade and can be bred to produce natural intergrades. You get the same effect if you take a red milk and cross it with an eastern. You could probably keep outcrossing for a specific look and have an "intergrade" that is more eye appealing than the natural occuring intergrades, but the people who are into locale specific stuff would rather have the natural occuring snake. The natural intergrades here can be rather dull looking, or very attractive!! Just comes with the territory when it comes to natural intergrades. Lots of different looking snakes when it comes to natural intergrades.
So it is just dependant on what you are looking for and if you want locale specific snakes. I like both. I enjoy the natural snakes we have here in KY and I also like the non-KY-native snakes that I have in my collection that are very attractive!!
Hope this helps
Brian
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RATS
1.0 Corn snake "Warpath" (KY locale)
1.0 Black rat snake "Havok" (KY locale)
1.1 Black rat snakes "Reaper and Mystique" (MO locale)
1.0 Albino Black rat snake "Malakai" (Dwight Good stock)
1.0 Everglades rat snake "Deadpool" (Dwight Good stock)
0.1 Greenish rat snake "Rogue" (Dwight Good stock)
1.0 Yellow rat snake "Wolverine" (Dwight Good stock)
1.0 Grey rat snake "Punisher" (White oak phase)(Dwight Good stock)
RACERS
1.0 Eastern Yellow Belly racer "Nightcrawler" (MO locale)
KINGS
1.1 California king snake "Bandit and Moonstar" (Coastal phase)
1.0 Prairie king snake "Bishop" (KY locale)
0.1 Black king snake "Domino" (KY locale)
1.0 Desert Kingsnake "Gambit"
0.1 Florida Kingsnake
MILKS
0.0.1 Eastern Milk snake "Cable" (KY locale)
0.0.1 Eastern/Red milk intergrade "Omega Red" (KY locale)
BULLS/GOPHERS/PINES
0.1 Sonoran Gopher
Good luck and Happy Herping
Brian