If your Web/SiteAdvisor icon is Grey and the checkmarks on search results are all Grey there are at least two possible causes.
1) - The servers may be down or otherwise unreachable. All communications from the SiteAdvisor program you're running go to the Site/WebAdvisor server complex in Santa Clara, California. It's very rare for the servers to crash, but sometimes they may go offline for planned maintenance or upgrades.
2) - It could be the connection to the servers which is the problem (could be DNS issues, ISP problem, general connectivity issues like TCP time-outs etc.).
In this situation, if you see the Site/WebAdvisor icon then it is running, but isn't receiving any data. Sometimes this happens only in Chrome, and that's because in Chrome Site/WebAdvisor is a third-party extension, not available through the Chrome Store, so a workaround is necessary to run it. In Chrome Site/WebAdvisor runs as a process (mcchhost.exe) which should be visible in Task Manager. If it's there, but isn't showing any site results, then Site/WebAdvisor is active but dormant.
If just one site shows grey in a search result, it means that site hasn’t been tested yet (a user can submit it for testing). Site submission is automatic but this web page may help: McAfee SiteAdvisor Software – Website Safety Ratings and Secure Search
Note: I've used the name Site/WebAdvisor throughout due to the name currently transitioning between the old SiteAdvisor and the new WebAdvisor.