Question: What is the best selling colour for HL Mountain Frame, 42?
SQL: SELECT IIF(SUM(IIF(T1.Name = 'HL Mountain Frame - Silver, 42', T2.SalesID, 0)) - SUM(IIF(T1.Name = 'HL Mountain Frame - Black, 42', T2.SalesID, 0)) > 0, 'Silver', 'Black') FROM Products AS T1 INNER JOIN Sales AS T2 ON T1.ProductID = T2.ProductID