SELECT 
  variant_id, 
  color 
FROM 
  cscart_product_feature_variants 
WHERE 
  feature_id = 549 
  AND variant_id IN (12537, 12532)

Query time 0.00039

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.0042687,
    "nested_loop": [
      {
        "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"],
          "loops": 1,
          "rows": 2,
          "cost": 0.0042687,
          "filtered": 100,
          "attached_condition": "cscart_product_feature_variants.feature_id = 549 and cscart_product_feature_variants.variant_id in (12537,12532)"
        }
      }
    ]
  }
}