It's only symbolic meaning, that's all. It can make things easier for the spellcaster if the symbol actually means anything to them, but then if a candle means nothing and they're just including it because the spell says so then it's decidedly un necessary.
I would say not, I use both coloured and plain candles as a way to focus my energy. They make it easier for me to relax and open up but if you do not need them then I would say you could remove them from the rituals.
Ritual tools are used to create "atmosphere", for example, you can't meditate while listening to rock, but if you listen to specific music that will help you. The same with candles, incenses, etc. It helps you to boost the ritual and feelings, but this does not mean that your work will not happen because there is something missing. Sometimes, you intention at the work and energy that you put in, is more important than the 'tools' used.
Candles can be used to your own liking, and can depend on what you feel will fall more to your need.
Candle importance really just falls on what you think. For certain things, it can be required. It is not necessary to include them however.
Also (DarkAdept), that's incorrect. You can listen to rock when meditating. I do it. It's a matter of what KIND of mediation you are doing. Not all meditation is just one strict void. The beat and chords can open your awareness to certain aspects of the music and in life, increasing the awareness in your psyche, so to speak.
I got the point, just to set right my comment, I was not speaking for specific situations (as I do with specific sounds, music for each working / meditation).
Generally speaking, before you develop your personal method, it is usually done with no sounds (focused in the breath for example), or WITH specific sound (drums, binaural beats, etc) which mean, music, incense or candle is not really necessary.