If every site did that then it would be harder to quickly spot one in a long list of tabs. A neat trick but I don't think it is a particularily good idea.

No, thank you. Make alternative favicons a standard, and let my browser handle it. Like the other comment says, if all of my icons went grey in a sea of 100 tabs, I'd be royally fucked. Google is already confusing enough with all their icons being basically the same.