The flying monkeys are probably my favourite thing in the book I wanted to feature it on a cover as a sort of Red Herring if someone was introduced to the story for the first time. You may also be wondering why there might not be any green on the cover like the city. Well did I mention anywhere that THAT is OZ? No, and again- if you were approaching the story for the first time, the fact that there was no green wouldn't really matter.
I DID play with using green in the rough development stages but I wasn't doing anything I was happy with. Green didn't fit the mood I was trying to set and the colours final colours I went with are a better fit for this design graphically than any green.