When you add a tag in your product page (ie: "rainbow"), it will be indexed to Algolia in the tags attribute.
When you add key:value tags, however, they will be indexed in the named_tags attribute, and the named_tags_names attribute will hold the property names.
If you would like to have your key:value tag values in the tags attribute, you would need to modify your products so that the tags only contain values and not the keys.