Is the whole cake black? It all depends on the type of icing too, I have often covered cakes with sugarpaste and painted on black or any dark colours needed as it takes an enormous amount of colouring to make white icing that dark. This usually only works for small areas, I wouldnt paint a whole cake black but have painted areas like roof of car (cake obviously) and the hair on Betty Boop type cake etc. For adults I would mix the very thick paste colouring that you can get in any of the places mentioned with some alcohol, vodka for light colours or any one for dark colours, the alcohol evaporates and the icing dries. It's not perfect but its better than eating all that colouring.