Relabeling 6-AWG Wire
The colors reserved for the grounded conductors (neutrals) are white and gray, and any base color with three continuous white stripes.  You can't re-identify white or gray insulated wires and use them for ungrounded conductors (hots) for circuits over 50V, unless they are a conductor in a cable or cord.  This is commonly seen when white conductors in NM (romex) or MC or AC cables are used as hots in switch loops.  

The code section to refer to is 


  200.7 Use of Insulation of a White or Gray Color or with Three Continuous White Stripes.  


So I'd just buy black for the hots, and relabel the one leg with red.