Android 7 Nougat Notifications - Why are notifications with gray icons in most cases?

[AC Question]

Only several applications appear with the original application icon color by a notification in Android 7 Nougat.

Is there any way to switch back to Android 6 Marshmallow like notifications? These were much better manageable, clear-cut.
If not, are there any plans for giving a possibility for change the design of notifications back to Marshmallow's notifications?