Though the use of novelty style, shaped candles has a longer history of usage in Hoodoo and Root-work, they are definitely gaining ground within the rest of the metaphysical community. Like other tools, they provide a deeper focus for ritual and spell work, thus enabling practitioners to produce more effective results. While someone with a background in Hoodoo may already know the intents and purpose behind a particular kind of shaped candle, those with a Wiccan or European-focused magical background may may be unfamiliar with their unique properties.
While many correlate the skull with death (and often terror), it has a much deeper connection to other aspects of the spirit world. Many European tribes used the skull to hold the essence of its owner, recalled in the Celtic tale of Bran the Blessed, whose head traveled and spoke with his companions long after death, and was eventually interred in a hill, to forever protect the land from invasion.
Considered to be the very seat of the soul, the place where individual godhood and person-hood resides, as well as your direct connection to the Divine, the skull symbol is often used to bring matters to "a head", to quickly resolve a situation, or to call upon Ancestors or patron deities for assistance in a difficult matter. While color correspondences are similar to other aspects of candle magic, they have a unique twist when applied to skull candles.Of course, every practitioner can (and should) have their own color associations, built by Tradition, study, and most importantly, individual practice and success. I strongly suggest that you use the color correspondences that have traditionally worked well for you, with a strong nod towards the unique properties of these very special candles.