Change Mapping of Existing Data failed

Elastic Stack Elasticsearch
1

I have changing my index use
curl -XPUT 'http://localhost:9200/index/_mapping/product' -d '
{
"properties": {
"imagecolor": {
"type": "string",
"null_value": "null"
}
}
}'
I check use curl -XGET 'http://localhost:9200/index/_mapping/product?pretty' my mapping succes but when I search my mapping not exist
My elasticsearch 2.3.1

2

What does it mean?

Can you provide us with a script that reproduces the issue?