With #6 you cannot re-mark. You can with #4 though, and #4 aluminum has the same ampacity as #6 copper at less cost.
You want NEC 210.5.
- By default you are not allowed to re-mark conductors at all.
- in multiconductor cables only, you are allowed to re-mark only neutrals to be only hots. This is not allowed for individual wires in conduit.
- for wires 4 AWG or larger, you are free to re-mark anything (except ground colors, 250.119) to anything else including ground.
So you have the right idea, but use #4 aluminum instead of #6 copper.
Don't get overexcited about distinguishing hot wires from one another. In North American split-phase, it usually doesn't matter. Black/black is fine. What's much more important is distinguishing each circuit from other circuits. I have one job where the installer installed four 240V circuits in one conduit: each is black/red and you can't tell em apart! Since there was another conduit parallel to that one, a better choice would've been 2 black and 2 red in each conduit.
Also, don't forget to use your all-metal conduit instead of the ground wire. No need for grounds in EMT or rigid, for instance.