traefik constraintes

docker-compose.blue.yaml

version: "3"
services:
  blue:
    image: containous/whoami
    labels:
    - traefik.http.routers.blue.rule=PathPrefix("/")
    - color=blue
  proxy:
    image: traefik:v2.5.4
    # Enables the web UI and tells Traefik to listen to docker
    command:
      - --providers.docker
      - --providers.docker.constraints=Label(`color`,`blue`)
    ports:
      - "80:80"
    volumes:
      # So that Traefik can listen to the Docker events
      - /var/run/docker.sock:/var/run/docker.sock

docker-compose.green.yaml

version: "3"
services:
  green:
    image: containous/whoami
    labels:
    - traefik.http.routers.green.rule=PathPrefix("/")
    - color=green
  proxy:
    image: traefik:v2.5.4
    # Enables the web UI and tells Traefik to listen to docker
    command:
      - --providers.docker
      - --providers.docker.constraints=Label(`color`,`green`)
    ports:
      - "81:80"
    volumes:
      # So that Traefik can listen to the Docker events
      - /var/run/docker.sock:/var/run/docker.sock