i really think evryone has a point on the composition of balck here. Godsryche is correct in saying that we should look at it from the series perspective at the auras seem to be light emanating from ones body. but i'm in the same page as SC it depends on how color is defined (especially in the series). I may have taken something Michael said out of context because I remember him saying that auric colors can be any color in the color wheel.
if we look at it (color/aura) from the light standpoint then essentially, say for black, everyone has a white aura but through the photon filtration is perceived in another color. if so how does it happen? because one has no control over ones auric color
Lucothea also brings up good point - what is aura and how does it make its color?