The first time a candle is lit, the wick will be white for a time during its burn-in period.  Afterward, it will be completely black throughout its life.  A well-cared-for candle should never have a smoke trail; smoke trails appear when candles are relit with rather long, unkempt wicks.  They don't last long as the flame burns but they don't look pleasing.  Tha's why you should clip the wick shorter before relighting a candle.