The power button blinks white when you first get into the car.
If you then press the power button without pressing the brake pedal it goes into accessory mode, which is basically just the infotainment screen and power for the USB ports. While in accessory mode the power button blinks red. Actually it doesn't so much blink as the brightness level undulates.
If you press the power button again without pressing the brake pedal it goes into ON mode, which turns on the instrument panel display, the DRL's, and also enables power windows and the fan (but not AC or heat). In ON mode the power button lighting is the same as accessory mode, i.e. it is red and it continues to undulate in brightness.
If you press the power button while pressing the brake pedal it goes into READY mode. The power button is still red but the brightness no longer undulates.
Note that while in Accessory or ON modes the electrical power is coming from the 12V battery and it will eventually drain it so you don't want to stay in these modes very long. If you will be parked for any length of time you should put it into READY mode so that all of the electrical power comes from the HV battery. Being in READY mode also allows you to run AC or heat.