Make browser tab flash a notification

For security reasons, my website automatically signs users out after 5 minutes of inactivity. I achieve this through jquery timeouts which are reset any time the user does what I consider an "activity". To ensure security, the timeout of the cookie is also set to 5 minutes, and my jquery performs a heartbeat back to the server to ensure the cookie doesn't expire.

Currently, at about 4 minutes of inactivity, a jquery ui dialog pops up, warning the user of their impending timeout. The user can choose to stay signed in, sign out now, or do nothing and they are forced to sign out at the end of the 5 minutes.

My problem is that I want to make the tab flash/blink with a different background color to warn the user that something is going on while they weren't paying attention. I'm just not sure how to go about doing this.