How to I troubleshoot my Time Capsule that does not show a green light?

Installed my Time Capsule in a new home. My service is xfinity which has built-in WiFi in the modem. Time Capsule also has WiFi activated. Both WiFi options work perfectly. However, my Time Capsule shows a yellow light rather than the green light, so I know that something is not right with the system. I've unplugged Time Capsule and rebooted -- does not resolve the issue. What is the troubleshoot protocol to solve the problem?