SELECT 
  variant_id, 
  color 
FROM 
  cscart_product_feature_variants 
WHERE 
  feature_id = 656 
  AND variant_id IN (1560, 1598, 1596, 6258, 6252, 1556)

Query time 0.00057

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "4.21"
    },
    "table": {
      "table_name": "cscart_product_feature_variants",
      "access_type": "range",
      "possible_keys": [
        "PRIMARY",
        "feature_id"
      ],
      "key": "PRIMARY",
      "used_key_parts": [
        "variant_id"
      ],
      "key_length": "3",
      "rows_examined_per_scan": 6,
      "rows_produced_per_join": 0,
      "filtered": "2.72",
      "index_condition": "(`softwarepirmam_hewadelivard_cscart_4`.`cscart_product_feature_variants`.`variant_id` in (1560,1598,1596,6258,6252,1556))",
      "cost_info": {
        "read_cost": "4.19",
        "eval_cost": "0.02",
        "prefix_cost": "4.21",
        "data_read_per_join": "190"
      },
      "used_columns": [
        "variant_id",
        "feature_id",
        "color"
      ],
      "attached_condition": "(`softwarepirmam_hewadelivard_cscart_4`.`cscart_product_feature_variants`.`feature_id` = 656)"
    }
  }
}