Gray icon Rated 4 out of 5 stars
What do you think if the icon is gray when no mail and red (as it should be) when at least one there? So it would be more similar to other user interface icons
You can customized your icons in every way by CSS. But you need some knowledge about addon and CSS. There is an explanation in CSS file of gmail watcher (You need to extract it from xpi file and need some more knowledge).