Casinos consider math, psychology and many other things when they create their product. Colour theory is definitely a huge part of their branding because when they build their brand, they have to carefully choose the colour. First of all, green colour is associate with money. It's also a calming color, it's not aggressive. As it has been said, Black is always elegant. Green and black is also a part of roulette. Blackjack, Poker and other casino game tables are also green, so some casinos stick with it to preserve the recognition. Green is also a positive colour and along with blue, it's associated with trust. You often see that right answers have green colour and wrong answers have a red colour. Green is also associated with healthy. There are many things going on behind the scenes.