Purple candles are all about power. First and foremost, they are associated with psychic power and any spiritual matters. Use them in rituals to enhance your magical abilities or spiritual awareness, such as opening your third eye, or experiencing astral travel. Any form of divination can be aided by burning purple candles.
Purple candles are also called for in workings that deal with power in the physical realm. Use purple candles in matters of ambition, wealth, and influence. Perhaps you wish to gain greater recognition in your community, or greater success in your job. Purple candles excel in cases such as these.
One final note: while other colors can (and should) be used for general healing spells, burning a purple candle is the best choice when you are fighting a serious, potentially deadly disease.
How should I charge my candles? When you are ready to use your candle, before you light it, take a moment to simply hold it in your hands and focus on your objective. In addition to the general intention, think about your situation in particular, and the change that you would like to create. Some people choose to anoint their candles with oil at this time, or carve symbols into the wax. That's entirely up to you.
Why should I trim my candle wick? The length of a candle's wick determines how much fuel (the melted wax) is feeding the flame. If the wick is left too long, the candle burns more quickly than it should. It also produces more soot as it burns, which can leave a residue on the walls and ceiling. If the wick is too short, the candle will not stay lit. The ideal length is between 1/8-1/4".