The LED notification light on the Nexus 60 which is located next to the front camera notifies you of the missed notifications. To enable notification light on Nexus 6P as well as the Nexus 5X, do as follows:
1. Open the Settings app
2. Under the Device settings, tap the Sound and Notification option
3. Under the Notification option, make sure to toggle on the Pulse notification light option to enable the notification light on Nexus 6P and 5X running the Android Marshmallow
** Note that, for now Google has not offered any customization options for the LED notification light and you have only the default options.