Black: meditation rituals, hex work, uncrossing rituals and spells to banish evil spirits and negative forces, rest, hidden matters, stops gossiping and meddling, and for any work invoking the Crone. Also for divination and protection.

Blue: magick that involves honor, loyalty, peace, tranquility, truth, wisdom, protection during sleep, astral projection and spells to induce prophetic dreams. Good for water magick as well.

Brown: spells to locate lost objects, improve powers of concentration and telepathy, and for protection of familiars and household pets. This is also used for grounding, stability, friendship, and earth magick.

Gold: spells that attract the power of cosmic influences, and rituals to honor solar deities. Also for attraction and prosperity.

Green: spells involving fertility, success, good luck, prosperity, money, rejuvenation and ambition, and rituals to counteract greed and jealousy, also for abundance, faith, and maturity. Can also be used for earth magick.

Grey: spells to neutralize negative influences. Glamouries.

Orange: spells to stimulate energy and motivation, kinship, kindness, warmth, work, career, gain, and opportunity.

Pink: love spells and rituals involving friendship or femininity.

Purple: spirituality, psychic manifestations, healing, and spells involving power, success, independence, and household protection.

Red: fertility rites, aphrodisiacs, passionate love, and spells involving sexual passion, health, physical strength, revenge, anger, willpower, courage, and magnetism. Also for fire magick.

Silver: spells and rituals to remove negativity, encourage stability, and attract the influence of the Goddess. Also for psychism, money, and moon magick.

White: consecration rituals, meditation, divinization, exorcism, and spells that involve healing, clairvoyance, truth, peace, spiritual strength, and lunar energy. You may also use white to substitute any color.

Yellow: spells involving confidence, attraction, intellect, healing, persuasion, communication, and air magick.

I am very aware that there are many more colors out there, but this is just a very small list of colors that I've compiled that are commonly used.