A "white lie" is an untruth that is considered to be trivial (ie. shouldn't have severe consequences) and is usually told with good intentions at heart. Some people tell white lies to be polite rather than hurt somebody's feelings. For example, your best friend paints a portrait of her parents and is very proud of it. You think it is terrible. She asks you if you like it, and you don't want to hurt her feelings, so you tell a white lie and say "Yes, it's lovely!".

Sometimes a white lie is necessary. Sometimes maybe we should express our opinion directly.

only for those who believe in "sin concept"

First, we should come to an agreement on what a "sin" means, before we can answer to your question.:)