Somebody who is Green / environmentally friendly on the outside, but Red / destructive on the inside. (Typically motivated by politics or environmental programs).
Can also be used to describe somebody who is kind / giving on the outside, but has cruel intentions hiding inside.
Politically green terminology
Their main concern is the environment, but in truth they are a watermelon; green on the outside, completely red on the inside.
Not only is it a delectible snack enjoyed by many people, but it was made into a (somewhat) racist joke by seventh graders from Long Island that nobody will ever understand including themselves.
“Hey mom, dont leave black shanta cookies, give him watermelon!”