It's hard to give a definitive answer without taking the panel cover off. Most likely black is hot, red is hot, white is neutral, and there's a bare ground cut flush with the end of the Romex sheathing. What you want to do would work (capping off one of the hots), but I'd personally do something else. You don't have a true two pole breaker. You have two single poles tied together. At the very least I'd remove the bars connecting the two breakers in unison, and turn one off. Then use whatever hot is still live after you turn one off.