Shades of Grey Require Black
The fact that we live in a world of relative good and relative bad, or, to use the cliche, myriad shades of grey, should not lead us to accept the conclusion that grey is all there is. Grey being the mixture of white and black, black, in this metaphor, absolute evil, exists as the a priori concept that brings darkness, evil, pain, what have you into the world.
(Of course, the possibility of this argument should not be taken as dispositive. It is equally possible that gray is a color, the only color, and there is no absolute a priori morality, only strategies based on contingencies responding to varying degrees of fellow-feeling, developed by experience, evolution, and temperament.)