20 But let us agree, as you wish, that there are both infernal regions and Manes, and that some gods or other dwell in these by no means favourable to men, and presiding over misfortunes; and what cause, what reason is there, that black victims, even of the darkest hue, should be brought to their altars? Because dark things suit dark, and gloomy things are pleasing to similar beings. What then? Do you not see— that we, too, may joke with you stupidly, and just as you do yourselves — that the flesh of the victims is not black, nor their bones, teeth, fat, the bowels, with the brains, and the soft marrow in the bones?
But the fleeces are jet-black, and the bristles of the creatures are jet-black. Do you, then, sacrifice to the gods only wool and little bristles torn from the victims? Do you leave the wretched creatures, despoiled it may be, and shorn, to draw the breath of heaven, and rest in perfect innocence upon their feeding-grounds? But if you think that those things are pleasing to the infernal gods which are black and of a gloomy colour, why do you not take care that all the other things which it is customary to place upon their sacrifices should be black, and smoked, and horrible in colour?
Dye the incense if it is offered, the salted grits, and all the libations without exception. Into the milk, oil, blood, pour soot and ashes, that this may lose its purple hue, that the others may become ghastly. But if you have no scruple in introducing some things which are white and retain their brightness, you yourselves do away with your own religious scruples and reasonings, while you do not maintain any single and universal rule in performing the sacred rites.
Source: Against the Heathen (New Advent)