How can I change the "You've got mail" messaging indicator envelope color? Is there a setting similar to as in allowing any indicators?
The indicator is not coded to be a specific colour, instead it switches from the
indicator-messages stock icon to the
indicator-messages-new stock icon.
ubuntu-mono-dark themes, the -new version happens to be blue. Other icon themes use different colors.
If you want, you can modify the icon svg from the Ambiance theme:
gksudo gedit /usr/share/icons/ubuntu-mono-dark/status/16/indicator-messages-new.svg
#19b6ee (the blue color used by default) and change it to the color of your choice.