One of the design decisions in the creation of the indicator menus was to have constant icons, rather than changing ones. In some versions, indicator-messaging will have a colour change when it detects new items, although this will not necessarily be present at login, depending on the interaction between the messaging provider and indicator-messages.
When a user opens the messaging menu, for any reason, the colour is adjusted to the base colour, even when the user has not viewed messages in all available applications: this allows the user to tell the system they are aware there are pending messages, and not be reminded to check again until more new messages arrive.
In the special case of liferea, there are some interaction issues related to parsing large numbers of feeds that can cause indicator-messages to be confused about the status.