When you get an error message, such as when trying to enable an extension that won't work, i.e.: "The selected extension is not available for this board, please verify your phpBB and PHP versions are allowed (see the details page).", the message is display in the green "Information" box.

Shouldn't these types of messages to the admin be in a red "Warning" box instead?

They are an indicator of a failed attempt by the Admin to do something in the extensions controller/manager, so should be in red, instead of the current green "success" style message box.