SELECT 
  variant_id, 
  color 
FROM 
  cscart_product_feature_variants 
WHERE 
  feature_id = 549 
  AND variant_id IN (9259, 9262, 9261, 9263)

Query time 0.00035

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_feature_variants",
      "access_type": "range",
      "possible_keys": ["PRIMARY", "feature_id"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["variant_id"],
      "rows": 4,
      "filtered": 75,
      "index_condition": "cscart_product_feature_variants.variant_id in (9259,9262,9261,9263)",
      "attached_condition": "cscart_product_feature_variants.feature_id = 549"
    }
  }
}