The Blue-Green Deployment is a technique for releasing your application in a predictable manner with the goal of reducing any downtime associated with a release.
The point is that at any given time, only one version of the application is serving client.
Contrast this with rolling-updates type of deployment.
It’s a quick way to prime your app before releasing, and also quickly roll back if you find issues.
http://blog.christianposta.com/deploy/blue-green-deployments-a-b-testing-and-canary-releases/