I take it you mean red hair as in someone who's a redhead, as opposed to someone who'd dyed their hair scarlet? In which case, I wouldn't think there was any "right" shade. Red hair varies widely from strawberry blond (gold with a hint of red) right the way to deep auburn (not far off chestnut brown). With lighter tones, you often find that the short hair near the scalp is darker than the outer hair, so which tone you choose will determine how much highlighting you need to get the effect. And another very important thing to remember (depending on scale, of course) is that eyebrows and lashes will be much the same colour.