"Her hair is black and nice."
If I'm not mistaken you're normally supposed to have adjective character numbers match for the you/you construction (i.e. youheise youhaoken or youhei youhao)...
No, that's unnecessary. It's still the same meaning to say 黑 or 黑色.
Also, youhei youhao isn't the same as youhei youhaokan.