MQTT Template light from 3 sensors

Configuration
1

I’m currently sending RGB data back to HASS using the following 3 sensors

- platform: mqtt
  state_topic: "sensors/RGB"
  name: "Red sensor"
  value_template: "{{ value_json.red }}"

- platform: mqtt
  state_topic: "sensors/RGB"
  name: "Green sensor"
  value_template: "{{ value_json.green }}"

- platform: mqtt
  state_topic: "sensors/RGB"
  name: "Blue sensor"
  value_template: "{{ value_json.blue }}"

What I would like to do is instead use my value_template entries for a single MQTT Template light (or similar).

That way, I can use the dynamic light icon to show me the actual color, rather than dealing with 3 separate numbers.

I tried this:

- platform: mqtt_template
  name: RGB Sensor
  command_topic: "sensors/RGB"
  command_on_template: "on"
  command_off_template: "off"
  red_template: "{{ value_json.red }}"
  green_template: "{{ value_json.green }}"
  blue_template: "{{ value_json.blue }}"

and while it gives me a new light, it doesn’t give me any colors.

instead I get:

2017-12-21 12:25:15 ERROR (MainThread) [homeassistant.helpers.template] Error parsing value: 'value_json' is 
undefined (value: on, template: {{ value_json.green }})
2017-12-21 12:25:15 ERROR (MainThread) [homeassistant.helpers.template] Error parsing value: 'value_json' is 
undefined (value: on, template: {{ value_json.red }})
2017-12-21 12:25:15 ERROR (MainThread) [homeassistant.helpers.template] Error parsing value: 'value_json' is 
undefined (value: on, template: {{ value_json.blue }})