The FAQ states the following;
What does the LED indicate when the PowerCore+ mini 3350mAh Portable Charger is charging itself or other devices?
The LED light will stay on when charging a device, it will be green when there is 50%-100% capacity left in the powercore; and then turn yellow when there is 20%-50% capacity left in the powercore; and turn red when the capcaity goes down to 5-20%. The red light will begin to flash when the capacity is lower than 5%.
When the powercore is being charged, a stead yellow light will stay on when the PowerCore+ Mini is charging. The yellow light will turn green when the PowerCore+Mini is 100% charged. The green light will turn off after unplug the PowerCore+ Mini.