Candles are a tool as is everything else in the spell, even yourself. It is felt by many people that having tools in a spell does increase the spells productability, and effeciency. The colors on a candle represent certain emotional states or physical objects to the caster, the colors meaning of which can be researched or discovered by yourself (or both). The color is determined by the intent or the desired outcome of the spell, not by the tools involved.
You do not need a specific type of candle to cast a spell. However, it is suggested that you use less synthetic candles, which I will leave up to your interpretation.