AWS Elastic Beanstalk will change the health status of a web server environment tier to gray color when:

A.
AWS Elastic Beanstalk detects other problems with the environment that are known to make the application unavailable.

B.
Your application hasn’t responded to the application health check URL within the last one hour.

C.
Your application hasn’t responded to the application health check URL within the last five minutes.

D.
Your application’s health status is unknown because status is reported when the application is not in the ready state.

Explanation:
AWS Elastic Beanstalk will change the health status of a web server environment tier to gray color when your application’s
health status is unknown (because status is reported when the application is not in the ready state).
http://docs.aws.amazon.com/elasticbeanstalk/latest/dg/using-features.healthstatus.html