Google it and look at babies, they are not all yellow. You need to do alot of research to make sure you are ready. A gtp is much different than a bp and a young gtp can be a handful. They are very delicate and many bite everything they can. Read up on the different locals and make sure you get the best for you. Many say an Aru is the most calm and the Biaks are the most nasty. I had a orange Biak that was the devil.
I moved and took my snakes. Will the snake not have to be moved from his place to yours? I'm wondering if the snake is not more than he signed on for? Just make sure you are getting what you want. Sometimes its better to get a sub adult or adult to have a better idea of what you are getting.
Nearly all Green Tree Pythons are green as adults. To what extent and pattern is a guess at best.