You should use a black candle for a warning spell. Candle magic is a form of sympathetic magic. It often employs black candles. Witches use them in rituals to manifest intentions related to transformation, protection, or banishing negativity.
In many traditions, black candles are used for protection and banishing negative energy. They absorb negative influences from other persons and ward off malevolent forces.
As you light a black candle, consider the centuries of history and meaning it carries.
Light a black candle and recite the following warning spell:
Blood turn black and flesh turn blue,
I will curse you if you force me to,
by the left hand and the unclean food,
I´ll curse your eyes, I´ll curse your lies,
I´ll call down a plague of flies,
blood go black and flesh go blue,
evil from me and back to you,
my soul clean and yours on fire,
you fuck with a witch you get burned, liar.