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You would need to do some software programming for this. Search arduino OTG RGB LED. You can set the color of an RGB LED using a phone's OTG port (uses the micro USB as a host) and sends serial commands to an arduino. At this point you might need to make a mod for the android OS that creates a virtual notification LED as part of the phone's hardware, and an app that spits out serial commands over USB based on what this virtual LED is doing.
Arduino Controlled CNC / 3D Printer Hybrid
Homemade 6 digits precision thermometer with Farenheit,Reamur,Kelvin and Celcius
Arduino True Battery Capacity Tester (Li-Ion/NiMH/NiCD/Pb)