YELLOW status in Elasticsearch

The status is yellow because we have unassigned shards.
In this case we have one node cluster, but our indices all have number_of_replicas: 1 (which is the default). So we have to add a second node to our cluster, or just set "number_of_replicas: 0" globally:
curl -XPUT -H "Content-Type: application/json" 'localhost:9200/_all/_settings' -d '{"index" : {"number_of_replicas" : 0}}'