The strength or potency of the spell or magick is increased ten fold if your candle is dressed with the proper oil.  Oils should be chosen for their purpose and not for their fragrance.  Some general purpose oils may include clearance, which clears on both the spiritual (psychic) and physical planes.  It may be used in conjunction with other oils as well to increase the effects of the ritual.  I have had great success with candles.  Mainly much is revealed to me through dreams and visions as a result of my regimented candle rituals.  High John the conquerer is an oil used to give courage and protection in difficult situations.  Once when I began a ritual, I dreamed that my candle stood as high as the clouds and stood in the east like the sun.  It burned out in all of one day to within one days oil, I always use the glass encased seven day candles.  When it reached the level of one day's oil it just kept burning and didn't get any lower, it just burned and burned.  When I awoke, my actual candle had burned only its normal amount.  Symbols can be drawn on the candles as well to attract specific spirits, or energies into the ritual.  The candles should also be properly incensed or suffumigated as well.  The hours of the day and the days of the week should be observed to get precisely the results you seek.
The planets rule the days and each has its own characteristic color that is associated with that planet.  If your ritual required a blue candle, then your ritual should begin on a Thursday, in the Hour of Jupiter to  infuse your operation with the proper energy.  Or if a red candle then your operation should begin on a Tuesday since red is the color associated with Mars, Tuesdays ruling planet.  etc.  There are many books and online grimoiries that contain the days and hours that the planets rule as well as a table of the magical names of hours.  There is also a characteristic stone and metal and angel for each day.  Ordering is necessary.  It is said that everything is interconnected from the largest solar body to the smallest grain of sand.  Ordering is done to restablish that interconnection and ensure success to any ritual.  So in essence you would have a candle, a metal, a stone, the name of an Angelic Being engraved on the candle, the day the hour, the oil, the incense etc........the associations and interweaving is endless.

that is strange i use black candles to expell evil spirits

Black candles does not mean evill per say. it can be attracting light. Black absorbs light. I use black if I want to rid negativity myself. Intent is the key of course I think the colors can mean what you want them to mean at time since certain societies see different colors in different perspectives.. example in china black is for weddings and white for furnerals.. you can make a color mean what you want it to mean. It does send out certain vibrations but my opionion the vibration you send out is far greater..

wow this is great I found the search box and my candles I was wanting to know more about. This is great information. Does the kind of candle really matter too? Like if they are parafin or wax? IM new here so if this is allready answered tell me and I can search for it thank you  

I was curious as to how candle use has been validated to really work and who actually started it? Im into fact finding and dont always rely on just cause someone says its true kinda thing. thanks