The indicator shows a little envelope which changes color if there are any notifications. If your head is turned and you miss the notification itself, the only evidence remaining of a notification is that the icon is a different color. If you have trouble seeing colors, distinguishing contrast, or just have an icon theme that doesn't have distinctive different colors, this means you're largely unable to tell if there are notifications waiting.
My suggestion would be either a differently shaped icon (maybe an envelope with a page poking out the top), a gray bubble to the right showing the aggregated count of new notifications (my personal preference), or both, or even an animation (perhaps a pulsing or a wiggle, similar to the behavior of Launcher icons for the urgent hint).
The messaging menu icon should be an open outline envelope when there aren't new messages, and a closed solid envelope when there are. Specification: <https:/