First, even without CustomUI you can use device_class to show a specialized on/off status and icon:
customize:
binary_sensor.door_back_sensor:
device_class: door # Also try window for another set of icons
If you want to use CustomUI to change icon color:
customize:
binary_sensor.door_back_sensor:
templates:
rgb_color: "if (state === 'on') return [200, 100, 30]; else return [10, 20, 30];"