Not all casinos that I've visited have any of the colors and I hope you mean the background layouts of the casino sites? However, those colors are just based on individual choices. It's just for the graphics which aims to provide users of the sites beautified and conveniences in glancing while the contexts and contents appears visibly readable and interesting in appearance.
It's one of the features that attracts the public to always visit. It's all just for marketing.
Some are mixed colors like white and red, green and black and so on with beautiful backgrounds on their colorful combinations.
So when you'd embark on setting up your own casino, you'll still have to consider them for glancing entertainment and interests of users.