You may not even realize it, but the latest Nexus devices have an LED notification light.
Google's new Nexus devices, the Nexus 5X and Nexus 6P, are both equipped with an RGB LED notification light. The small light is a convenient way to remind or alert you of pending notifications.
However, you'll need to enable the small blinking light before you can begin using it.
Enabling it takes a few taps:
When enabled, new notifications will cause the light to blink. Unfortunately, there doesn't seem to be any sort of native tool built into Android 6.0 Marshmallow that allows you to customize the alerts.
Thankfully, you can use an app such as Light Flow to do just that. I've installed the free and paid version of the app and have success in customize the frequency of the light's pulse and color.
On the 5X, the light is found beneath the screen in the speaker grille. On the 6P, it's found in the top-left corner, next to the front-facing camera's lens.