Animal emotional response to the environment is often more complex than what we are used to believe. Dogs can fall in love, and suffer from it if they are not corresponded, so I would not be surprised if apes could understand fairness. But I believe that further tests are necessary before we can take this theory as a fact.
Does this test really show the monkey's ability to comprehend fairness or just preference? I would need to know if there was a test done before where only the green one was offered.