Mystery
Secrets
Invisibility
Illusion
Neutrality
Grey is a cross between white and black, making it a good colour to chose when looking for candles to represent neutrality and balance. They are good for spells of protection. They are also useful for use in meditation regarding complex issues. They can be used in spells to repel negative energy. Grey candles can be used in spells for unveiling secrets, as well in meditation for revealing hidden issues that we feel yet don't necessarily understand.
Grey candles are often used in place of silver candles, but their uses should not be confused. Of course, the power of the witch is a big factor, however, using the wrong colour an affect the affectiveness of the spell. Think of it as using purple confetti instead of lavender buds. It ups the level from just spellcasting to also substitution and imagination. You have to both make sure that the representation remains appropriate for the spell and also convince yourself that the substitute will work. A major part of spell casting is belief, so if you don't believe that the spell will work, it most likely won't, meaning that if you use substitutes, you have to make sure that you believe in their ability to work.