[@Cenco] In the OP it would explain that each color has a specific characteristic, with the exception of black & white.
It seems like you have a character in mind that involves shapeshifting. Instead of an [i]unknown[/i] color, you could have your duel avatar color be either black or white, seeing as they dont have any specific characteristics.

Being a black/white avatar, opens the posibilities to have unique traits(such as shapeshifting). 

[@Vocab] I honestly thought Void was white O.o XD