Super Saiyan 6 is the sixth form of the Super Saiyan forms. To obtain it, the user needs to work against their tranquility obtained from their Super Saiyan 5 form, which they must train as. The form overall focuses on brute strength and power to out-class the opponent. The dark color of the fur and hair reflects the violence and power of the user.
Where Goku was able to achieve Super Saiyan 5 easily due to it's peacefulness, Vegeta made up by achieving this form faster, due to his more violent and impatient nature.
This form can only be acheived if in a Fusion, such as Gogeta, as no single being can bare the power of it. And through training that put's a living being's body to the absolute limits, they can acheive a far more powerful form...